Add a "Disconnect" action on the ongoing peer connectivity notification (4.1+ only).

Utilize additional action (button) that can be attached to the peer connectivity notification to disconnect (4.1+ only). Currently, to disconnect from the network, the user has to launch the application's main activity first and then tap the "Disconnect" menu item from the overflow. Using an action button on the notification itself is much more efficient.

Looks like this:

![screenshot_2014-01-28-13-38-55](https://f.cloud.github.com/assets/152710/2024113/47c4a94a-8865-11e3-9df0-5e602ecd35aa.png)


However, this requires android-v4 support library r11+, whereas the current maven configuration pulls from support library r7. I had to comment that out and replace the support library manually (r13 is bundled with the Android SDK). I don't see any maven central repository for this, actually (seems like the ones that are there are not official to begin with).