FSEvents on macOS

Requested by @robfig at https://github.com/howeyc/fsnotify/issues/54 in order to watch large trees.

To be implemented as a stand-alone repository for experimentation (#23) before determining how to incorporate it into fsnotify.

System requirements:

• OS X 10.6 or better (matching the system requirements of Go 1.3). Though some features require 10.7, or perhaps even higher.
• Likely require cgo with Xcode installed.

Prior work:

• @samjacobson https://github.com/samjacobson/go.fsevents
• @ronbu https://github.com/ronbu/fsevents

Reference:

• Advanced Mac OS X by Mark Dalrymple (@markd2) contains a chapter on FSEvents
• FSEvents Reference
• ArsTechnica