File notifier

Option to write Guard notifications to a file. Each notification will overwrite the file. This allows other commands to be run based on the status of other guard commands.

I tried hard not to say "meta-guard" in the README.

Please see this relevant discussion: https://groups.google.com/forum/?fromgroups=#!topic/guard-dev/wz5zlLJUEQA.

One thing to note. I wanted to disable the notifier when no `path` option is provided. Right now that happens in [`.notify`](https://github.com/amiel/guard/blob/file_notifier/lib/guard/notifiers/file_notifier.rb#L41) as `.available?` doesn't have access to the notification options.
Since this is still an internal api, it seems like it could still be possible to change `.available?(silent = false)` to `.available?(options = {}, silent = false)`. Thoughts?