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.
MPD.java: Improve Add, Remove & Play handling (from 6d0141a).

Before this patch, if the user selects Add, Remove & Play (ARP):

If consume mode is enabled, it chews up the first song (whoops!).

If random mode is enabled we would play the second song on the playlist queue,
which is unexpected behaviour, so we now send a next command in this case.

If Add & Play (not removing), and random mode is enabled, we would play:
(old playlist queue+1) which is flawed thinking, as in random mode it will
revert to the old playlist queue at any point afterwards, so, in this case,
play doesn't make sense, so we now toast the user that we don't Add & Play
in random mode.