MPDControl: Add a new helper to send simple commands to MPD.

Remove the command code from NotificationService and add it to a new class
MPDControl. Add code to overload the method for resource ids, and other useful
stuff. Also, clean up the now duplicate code.
MPDroid: Cleanup notifyUser() use.

This cleans up notifyUser use and adds a new
method which takes a resource id to further clean things
up.
MPDroid: Remove redundant calls to String.Format() when calling getString().
MPDroid: Add an overload for notifyUser() to call getString() with resId.
MPDStatus: If mixRampDB equals 'nan' set mixRampDisabled.

If mixRampDB equals 'nan' (Not A Number) set mixRampDisabled. This fixes #547.
MPDStatus: Add mixRamp accessors.
MPDStatus: Move mixRampDelay to reset everytime, as it's not always shown.
MPDroid: Omit the getString() call when the method accepts resources.
NowPlayingFragment: Remove unused method.