Handle symlinks

[smashwilson]

Distinguish and report events relating to symlinks.

• Introduce a kind: "symlink" for filesystem events that involve symlinks. (Symlink entry kind #111)
• When a watch root path containing a symlink is passed to watchPath(), report paths to listeners as the symlink path, but disambiguate the watch root using the realpath. (Report paths consistently with watcher creation #113)
• Include a test to be sure we don't lock on up symlink loops. We shouldn't because of the way I'm traversing the filesystem but it pays to be sure. (Unit tests for symlink edge cases #114)
• Include a test for handling broken symlinks gracefully. (Unit tests for symlink edge cases #114)

[smashwilson]

Notice that this does not include support for followSymlinks. I've broken that out to #112 for clarity.