Google Play refuses uploads with a lower targetSdkVersion.

add 2 preferences to:
 - enable Wakelock when MPD is running (prevent suspend)
 - run MPD on boot

and display MPD logs

 - <uses-permission> must be before <application>
 - specify allowBackup (default)

- add Settings: Activity to start / stop MPD Service (Main).

- Main is a service that run in foreground with a notification. See
  Service.startForeground documentation for more details.

- Main.Client is used to control the service: start or stop it and also receive
  callbacks when service encounters an error, is killed, is started or is
  stopped.

- Main.start to start the service without any fallback.

The number of MPD installs on Android < 5.0 is negligible, and that
API version introduces lots of useful features for MPD.

Closes #213.