Rename Phase -> Goal.

Review Request #887 - Created Aug. 13, 2014 and submitted

Information
Benjy Weinberger
pants
Reviewers
pants-reviews
jsirois
This change deals with phase.py (now goal.py) and things that
depend on it.

Future mop-up changes will fix extant uses of the term 'phase'
in various files, and also fix documentation appropriately.
CI passes: https://travis-ci.org/pantsbuild/pants/builds/32484729
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