EMFILE error while watching large directories on OS X

[Aintaer]

As seen in issue #1, watching a large directory/many subdirectories runs into a EMFILE error. This error is thrown by node when attempting to call fs.watch() after all file descriptors allotted per-process runs out. This is usually not an issue, but OS X sets a default per-process file descriptor limit of 256.

Node has been aware of this issue, see nodejs/node-v0.x-archive#2479

[Aintaer]

The only solution that would suffice, save a completely rewrite of lib/watcher to split into multiple processes, is the one increasing the file descriptor per process limit on OS X nodejs/node-v0.x-archive#2479 (comment)

Otherwise, I'll mark this for 0.3 to handle multi-process watching.