Symlink entry kind

[smashwilson]

Add an EntryKind to represent actions performed on symlinks. Identify symlinks from uv_fs_lstat() and uv_fs_scandir() results and report them as a distinct kind in reported events.

Part of #19.

[smashwilson]

Grr. inotify only includes a boolean IN_ISDIR flag on events. While the only two entry kinds were ENTRY_FILE and ENTRY_DIRECTORY, that was enough to classify events properly, but it isn't enough to identify ENTRY_SYMLINK.

I'm going to introduce a RecentFileCache into the Linux handler and use it to balance the cost of doing an lstat() on every path with the cost of storing a bunch of previous results. That's complex enough that I probably should work on an actual Linux machine to do it, though.