Added an excludes attribute that is set to an empty set for all SourcePayload subclasses.

See github issue https://github.com/pantsbuild/pants/pull/414.

Testing Done:
ci passed @ https://travis-ci.org/pantsbuild/pants/builds/31232287

pants goal idea src/thrift:: brings up the IDE and puts the source dirs on the content root.

Bugs closed: 414

Reviewed at https://rbcommons.com/s/twitter/r/793/