Lollipop lock screen notifications

Add support for lock screen notifications on Lollipop.

Vectorize the icons used there (hooray Illustrator).
Bump SDK and support-v4 to 21.0 to get lock screen notification API support.

No changed for notifications on pre-Lollipop devices.
Changing `targetSdkVersion` can change the behavior of system components. Did you check the changes to make sure we're not affected by this?
http://developer.android.com/about/versions/android-4.3.html
http://developer.android.com/about/versions/android-4.4.html
http://developer.android.com/about/versions/android-5.0.html