Notification and remote client

Hi,

Here's a feature that I found to be missing in the app when I downloaded it from the Play Store.

It's basically just a notification (incl. rich notification when pulling with two fingers) and a remote client (e.g. lockscreen widget) that allow to quickly:
- review current played item (artwork / artist / album / title)
- control MPD (prev/next/play-pause/stop)

It's currently:
- not 100% functional (there are some cases where the notification/remote client aren't displayed while MPD _is_ playing)
- not 100% tested: only used with my development device, which is a Nexus 5
- not 100% final: there are quite some debug logging made through the logcat

These numbers, while not being 100%, are quite close to it.

I might test on 4.0 emu or Nexus 7.

It is based off of my gradle branch, because I use Android Studio and gradle to build the app. I don't know git/github enough to submit a clean set of commits that only contain the notification/remote client features. I hope ant/eclipse users will be able to merge this in order to test! (see #402)