Allow project Resource Path Variables to be used in PythonPath string substitutions

For example, `PROJECT_LOC`, etc.

This allows paths relative to project and workspace locations to be more easily added. For example, this is desirable when developing a project using buildout in a virtualenv, particularly one with a complex `PYTHONPATH` such as Plone.

Also requested as a feature in [this earlier forum post](http://sourceforge.net/projects/pydev/forums/forum/293649/topic/3822328).

It's been a while since I wrote Java, so code and style criticism is most welcome.