Add a CI action to support CI against OSX.

This adds a sync action run on any push to pantsbuild/pants
master that replaces pantsbuild/pants-for-travis-osx-ci master
modifying the active .travis.yml to trigger an OSX CI run.

Testing Done:
Normal CI went green here: https://travis-ci.org/pantsbuild/pants/builds/35900516

OSX CI went green here: https://travis-ci.org/pantsbuild/pants-for-travis-osx-ci/builds/35899630
Via:
```console
$ set | grep GH_ | cut -d= -f1
GH_EMAIL
GH_TOKEN
GH_USER
$ ./build-support/bin/ci-sync.sh
[jsirois/pants-for-travis-osx-ci/sync 9a13170] Prepare pants OSX mirror for CI.
 Author: Pants CI <pants-ci@googlegroups.com>
 1 file changed, 13 insertions(+), 40 deletions(-)
 rewrite .travis.yml (87%)
Counting objects: 37, done.
Delta compression using up to 8 threads.
Compressing objects: 100% (18/18), done.
Writing objects: 100% (23/23), 3.05 KiB | 0 bytes/s, done.
Total 23 (delta 14), reused 9 (delta 3)
To https://github.com/pantsbuild/pants-for-travis-osx-ci.git
 + 888c8bf...9a13170 HEAD -> master (forced update)
$ gitbinfo
* jsirois/pants-for-travis-osx-ci/sync 9a13170 [pants/master: ahead 2] Prepare pants OSX mirror for CI.
$ git slog -2
commit 9a13170039dfe93824fd9487f78f96c134e09d88
Author: Pants CI <pants-ci@googlegroups.com>
Date:   Sun Sep 21 20:22:52 2014 -0600

    Prepare pants OSX mirror for CI.

 .travis.yml | 35 ++++-------------------------------
 1 file changed, 4 insertions(+), 31 deletions(-)

commit f4cb72ba4cc48ec5f6a23c86903fe83b735a8f3f
Author: John Sirois <john.sirois@gmail.com>
Date:   Sun Sep 21 17:30:29 2014 -0600

    Add a CI action to support CI against OSX.
...
```

Bugs closed: 596

Reviewed at https://rbcommons.com/s/twitter/r/1058/
Sync the OSX CI eagerly and add diagnostics.

Move the OSX CI sync up-front to leverage parallelism
instead of waiting on the whole local build matrix to complete.

Also add diagnostics that record why the sync is skipped if
skipped in addition to making skipping & syncing decisions more
robust and explicit.

Testing Done:
CI away here already shows proper fork sync-skipping diagnostic: https://travis-ci.org/pantsbuild/pants/builds/35967008
This will have to land on pantsbuild/pants master to vet fully.

Bugs closed: 599

Reviewed at https://rbcommons.com/s/twitter/r/1064/
Fixup ci-sync.sh to cover committer ident.

Testing Done:
None - can only be tested on pantsbuild.pants master, but in response to this failure: https://travis-ci.org/pantsbuild/pants/jobs/35969933
CI did go green here though - so nothing horribly more broken: https://travis-ci.org/pantsbuild/pants/builds/35972662

Bugs closed: 600

Reviewed at https://rbcommons.com/s/twitter/r/1065/
Prepare pants OSX mirror for CI.