Watching multiple directories

Thought I'd give a shot at making multiple directories work. All you need to do is separate the directories you'd like to watch with the space (e.g. `guard -w ~/path1 ~/path2`).

Note, this patch still passes along all added, modified, and deleted paths as relative paths, so it isn't very easy to discern which watched folder the change came from. But we could get around that pretty easily by adding other option to guard that also passed along the watched directory name (e.g. `path1/dir/changed.txt` instead of `dir/changed.txt`)

Please let me know if there is anything else I should be doing and/or testing for this PR.