always set base directory for bndlib Builder

Hey @TomDmitriev 

After a few months absence I'm back working with your gradle-bundle-plugin again.  Just so you know I'm working on migrating a large collection of bnd projects that current use ant scripts to drive the build and I'm one-by-one trying to move them to gradle your gradle-bundle-plugin.

Thanks so much for your work on this so far, its really good stuff.

So the most recent snag I hit I posed as #22 

This pull request fixes that issue by setting the base directory of the bndlib Builder for each project so multi-project builds or "working dir outside of project dir" works as expected.