Add support for explicit absolute target address sepcs.

This provides a clear  way to refer to root level targets
from deeper in the BUILD tree.

Testing Done:
$ PANTS_DEV=1 ./pants goal test tests::

travis-ci went green here: https://travis-ci.org/pantsbuild/pants/builds/28347405

Bugs closed: 269, 272, 278

Reviewed at https://rbcommons.com/s/twitter/r/590/