Fix SoundEffect.Play return value and iOS/Mac crash

Currently on iOS if you try play too many sounds then SoundEffectInstance will throw an InstancePlayLimitException, this differs with XNA behaviour.
Also, currently SoundEffect.Play always returns false.

This patch fixes both of these, we add an internal TryPlay method to SoundEffectInstance which returns false if the sound did not start playing (Only possible on iOS and Android) and use this in SoundEffect.Play to match XNA behaviour.
On other platforms we always return true as we don't seem to be able to fail to start playing.

ref: http://msdn.microsoft.com/en-us/library/microsoft.xna.framework.audio.soundeffect.play.aspx