Reintroduce fsevents_file_watcher.py

This patch makes fsevents_file_watcher the default again on Mac. It was
removed in the 1.8.x SDK versions. To use the mtime_file_watcher
instead, invoke "dev_appserver.py --use_mtime_file_watcher=yes"

You will need pyobjc installed to use the fsevents file watcher.
Running "pip install pyobjc" should do the trick.

The old file watcher was resurrected from version control at
https://code.google.com/p/googleappengine/source/browse/trunk/python/google/appengine

It's easiest to apply this patch using `git am <this.patch` or `patch
-p1 <this.patch` after having already applied watcher-skip-files.patch