The UseUse::$alias node can now be null if an alias is not
explicitly given. As such "use Foo\Bar" and "use Foo\Bar as Bar"
are now represented differently.
The UseUse->getAlias() method replicates the previous semantics,
by returning "Bar" in both cases.
The parser will now always generate Identifier nodes (for
non-namespaced identifiers). This obsoletes the useIdentifierNodes
parser option.
Node constructors still accepts strings and will implicitly create
an Identifier wrapper. Identifier implement __toString(), so that
outside of strict-mode many things continue to work without changes.
Adding this as an option to avoid breaking people's tests.
Some of the test results show pretty clearly that we are incorrectly
assigning the same comment multiple times for nested nodes (mentioned
in #36).
