Add support for the Wire protobuf library.  Adds the java_wire_library() target.

https://github.com/square/wire

Testing Done:
Added an example, plus new unit and integration tests.
CI is running @ https://travis-ci.org/pantsbuild/pants/jobs/39988400

Bugs closed: 765

Reviewed at https://rbcommons.com/s/twitter/r/1275/