Build gerrit-owners plugin against gerrit 2.8
Build gerrit-owners-autoassign plugin against gerrit 2.8
Remove pom.xml files since now gerrit builds with buck.
Fixes to buck build.

I have restructured the build files to ensure that both clean builds and incremental builds work.

Looks like buck has a couple of issues:
- it's "illegal" to have multiple gerrit_plugin targets in a single BUCK file,
  i.e. their MANIFESTs will silently collide.
- it's illegal to create gerrit_plugin targets with no source files.
Merge pull request #3 from ugodiggi/master

Update gerrit-owners* plugins for gerrit 2.8