Clean up middleman-livereload

* Use the correct file ignore API - we don't really care if the file is ignored by the sitemap (actually, we still want to reload when sitemap-ignored stuff is changed, that makes stuff like partials work!), instead use the (not yet public) `FileWatcher#ignored?` method which is what filters out stuff like `.git` and `.DS_Ignore` and emacs tempfiles.
* Remove `grace_period` hack. I'm not sure what this was trying to do - the sitemap is synchronized and must be updated before a request can go through, so waiting buys you nothing. Same with the `ensure_resource_list_updated!` calls - they didn't do anything. Furthermore, `grace_period` was hanging the thread with `sleep`, which actually *could* cause other parts of Middleman to hang up.
* Use the logger instead of bare `puts`
* Stop using `MultiJson`, require Ruby 1.9.3.