Export watch_file.

Export watch_file, as this is the easiest API entry point between FileMonitor
and PollingFileMonitor.

Fix _uv_hook_fseventscb such that the condition is notified even if no callback
is in the monitor. Also fix the value sent to notify() to match the expected
return value in wait().

Fix watch_poll default callback.
Document watch_file method.
Add a test for wait(FileMonitor).
Unexport new UV_ constants.
WIP: File watching event refactor.
Re-passing tests.
Refactor events with integer flag fields and accessors.
Fix logic bug in FDWatcher wait()
Merge branch 'watch-refactor' of https://github.com/blakejohnson/julia into jb/watch-refactor
my version of the file watching API refactor
Merge branch 'master' of github.com:JuliaLang/julia into jb/watch-refactor