Support absolute paths for spec_paths when they start with the build root and are supplied on the command line.

As proposed and discussed here: https://groups.google.com/d/topic/pants-devel/scbY4MM1XdM/discussion

Testing Done:
$ PANTS_DEV=1 ./pants goal test tests/python/pants_test/base/:cmd_line_spec_parser

Bugs closed: 471

Reviewed at https://rbcommons.com/s/twitter/r/867/