added ofxiOSSoundPlayer getAVSoundPlayer() method.

ofxiOSSoundPlayer getAVSoundPlayer() returns a pointer reference to the
native AVSoundPlayer object. this is the same as in the
ofxiOSVideoPlayer.
fix incorrect doc for ofRandomWidth()/Height()
Merge pull request #2959 from admsyn/fix-dox-random-width

Fix incorrect documentation for ofRandomWidth()/Height()
Merge pull request #2953 from julapy/ofxiOSSoundPlayer_getAVSoundPlayer

added ofxiOSSoundPlayer getAVSoundPlayer() method.
Merge branch 'master' of https://github.com/openframeworks/openFrameworks.git into rexgo