Added ability to play sound when message is displayed

Allows users to set sounds for use when displaying messages.

Users can set a single sound for use with all messages by calling:
`+ (void)setNotificationSoundWithName:andExtension:`

...or set a sound for use with a specific `TSMessageType` by calling:
`+ (void)setSoundWithName:extension:forNotificationType:`

If no sound is set for a particular `TSMessageType`, but **is** set with 
`+(void)setNotificationSoundWithName:andExtension:`, the latter will be played as a fallback. If there is nothing to fallback to, then no sound is played.

I look forward to hearing your thoughts and opinions on this.