added ofxiOSSoundPlayer getAVSoundPlayer() method.

ofxiOSSoundPlayer getAVSoundPlayer() returns a pointer reference to the
native AVSoundPlayer object. this is the same as in the
ofxiOSVideoPlayer.
Merge pull request #2953 from julapy/ofxiOSSoundPlayer_getAVSoundPlayer

added ofxiOSSoundPlayer getAVSoundPlayer() method.