Fixup the PrepareResources task to properly augment the classpath in
the exclusives regime.

Undo pingpong example workaround.

Perform minor cleanups on exclusives related code read to grok this
fix.
Merge pull request #171 from jsirois/jsirois/pants/issue_160

Fixup the PrepareResources task to properly augment the classpath in the exclusives regime.