Notifications (for linux too!)

h RoxasShadow.
I worked a bit on notifications: TerminalNotifier is mac-only, so I couldn't run it on my xubuntu dev vbox.
To address this problem (and achieve cross-platform notifications) I added a new class Notifier which spawn (with Notifier#getNotifier) the correct notifier according to the OS Miyuki is running on.
Actually, only Linux and Mac OS X 10.8+ are supported, take your time to investigate Windows alternatives.

A thing it could be ok to work on should be installing only the right dependency (TerminalNotifier on mac, libnotify on linux, libwhatever on windows) and include it in lib/miyuki.rb, instead of downloading 3 or more libraries and use just one of those.

This PR also fixes a bug in the comparison of torrents (to check if there are new torrents), but in the wrong way. The best way is to address this problem in Yamazaki::Torrent and override "==" operator. I would have been more than happy to provide it myself with a PR on Yamazaki, but I wasn't able to test it.