#22 allow specification of working directory

[Nikolay Metchev]

It is desirable to seperate the .g files from the
generated code. If you have multiple .g files which
depend on each other by using importVocabs then it is
not possible to keep the .g files seperate from the
generated source.

This is because importVocabs are only searched for in
the current directory.

I can think of one workaround for this limitation. In
the individual properties of a .g file it would be nice
to allow specification of the "working directory" from
where the antlr tool will be run.

[Kristof Szabados]

Fixed.
The latest version allows the specification of the imported vocabulary grammar.
Plus both the import vocabulary and super grammar attributes are used to create a fail-safe build ordering of grammar files.