Add support for FSEvent notifications

[jamesarosen]

This will save on battery life and hard drive wear on OSX >= 10.5 systems. It should degrade cleanly on systems where FSEvents are unavailable.

[mynyml]

I would love for this feature to be added to watchr, but I don't own a mac so can't add it myself. I'd really appreciate if someone were to provide a patch.

Meanwhile, the Rev event library uses kqueue on bsd so OSX users are not left out. I don't know how kqueue compares to FSEvents performance-wise though.

[sandro]

My fork has a really dirty patch which adds ruby-fsevent to the library. Now that I see the author has some interest in supporting this feature I may try giving it another go. Overall, it'd be great if we could enable fsevents without the rubycocoa dependency.

[mynyml]

sandro, I pushed your changes to the fsevents branch to confirm the interest, but I can't do much more since I'm not able to run the handler. I'll keep merging changes in the branch, and will merge into master when it's ready (functional and cleaned up).

[mynyml]

I finally have a mac to play with. I've updated/cleaned up the fsevents branch. It still has a few quirks (reported event aren't always detected), but is already fairly usable. If someone has a bit of time, I would greatly appreciate feedback before I merge into master. Thanks.