Added ability to set a default sound to use when message is presented.
Added ability to set default sound for each `TSMessageType`.
Updated `.podspec` to include `AVFoundation.framework`.
Updated `Example.xcodeproj` with demonstration.

Created TSMessage+Sounds category to extend TSMessages.
Category includes `TSMessageSoundPlayer` object that responds to
NSNotification.
Category header is imported inside `__has_include` macro in
TSMessageView+Private.
Added `TSMessages` as a subspec in `.podspec` and set to
`default_subspec` so its always included.
Added `Sounds` subspec in `.podspec` that includes the relevant files
and frameworks, with a dependency on the `TSMessages` subspec.

Added TSMessage Delegate protocol.
Added willDisplayNotification: delegate method.
Added didDisplayNotification: delegate method.
Removed all static variables from TSMessage+Sounds.
Condensed TSMessageSoundPlayer class.
Added implementation examples in Example project.

Added documentation for TSMessageDelegate, TSMessageSoundDelegate, and
all optional methods.