Add a filesystem template loader.

The template loaders loads templates from a base directory (and can
optionally use a suffix). For example

    $templateLoader = new PHPParser_TemplateLoader(
        $parser, './templates', '.php'
    );

    // loads ./templates/TestTemplate.php
    $templateLoader->load('TestTemplate');

Again the implementation is not optimal. The loader probably shouldn't
intantiate the Template itself, but instead should accept a
TemplateFactory. This seemed like overkill to me, so I left it out.

lib/PHPParser/TemplateLoader.php

@@ -0,0 +1,48 @@
+<?php
+
+class PHPParser_TemplateLoader
+{
+    protected $parser;
+    protected $baseDir;
+    protected $suffix;
+
+    /**
+     * Constructs a filesystem template loader.
+     *
+     * The templates are loaded from {baseDir}/{name}{suffix}.
+     *
+     * @param PHPParser_Parser $parser  A PHP parser instance
+     * @param string           $baseDir The base directory to load templates from
+     * @param string           $suffix  An optional suffix to append after the template name
+     */
+    public function __construct(PHPParser_Parser $parser, $baseDir, $suffix = '') {
+        if (!is_dir($baseDir)) {
+            throw new InvalidArgumentException(
+                sprintf('The specified base directory "%s" does not exist', $baseDir)
+            );
+        }
+
+        $this->parser  = $parser;
+        $this->baseDir = $baseDir;
+        $this->suffix  = $suffix;
+    }
+
+    /**
+     * Loads the template with the specified name.
+     *
+     * @param string $name The name of template
+     *
+     * @return PHPParser_Template The loaded template
+     */
+    public function load($name) {
+        $file = $this->baseDir . '/' . $name . $this->suffix;
+
+        if (!is_file($file)) {
+            throw new InvalidArgumentException(
+                sprintf('The file "%s" does not exist', $file)
+            );
+        }
+
+        return new PHPParser_Template($this->parser, file_get_contents($file));
+    }
+}

test/PHPParser/Tests/TemplateLoaderTest.php

@@ -0,0 +1,44 @@
+<?php
+
+class PHPParser_Tests_TemplateLoaderTest extends PHPUnit_Framework_TestCase
+{
+    public function testLoadWithoutSuffix() {
+        $templateLoader = new PHPParser_TemplateLoader(
+            new PHPParser_Parser, dirname(__FILE__)
+        );
+
+        // load this file as a template, as we don't really care about the contents
+        $template = $templateLoader->load('TemplateLoaderTest.php');
+        $this->assertInstanceOf('PHPParser_Template', $template);
+    }
+
+    public function testLoadWithSuffix() {
+        $templateLoader = new PHPParser_TemplateLoader(
+            new PHPParser_Parser, dirname(__FILE__), '.php'
+        );
+
+        // load this file as a template, as we don't really care about the contents
+        $template = $templateLoader->load('TemplateLoaderTest');
+        $this->assertInstanceOf('PHPParser_Template', $template);
+    }
+
+    /**
+     * @expectedException InvalidArgumentException
+     */
+    public function testNonexistentBaseDirectoryError() {
+        new PHPParser_TemplateLoader(
+            new PHPParser_Parser, dirname(__FILE__) . '/someDirectoryThatDoesNotExist'
+        );
+    }
+
+    /**
+     * @expectedException InvalidArgumentException
+     */
+    public function testNonexistentFileError() {
+        $templateLoader = new PHPParser_TemplateLoader(
+            new PHPParser_Parser, dirname(__FILE__)
+        );
+
+        $templateLoader->load('SomeTemplateThatDoesNotExist');
+    }
+}