Refactor JSQMessage object hierarchy?

[jessesquires]

Investigate if JSQTextMessage and JSQMediaMessage are still necessary.

Might make sense to re-combine under JSQMessage.

[jessesquires]

Because of the later addition of isMediaMessage, these 2 subclasses should be absorbed into the parent JSQMessage class.

This flattens our view hierarchy and removes ambiguity and data duplication. (e.g., should you check isMessageMessage or isKindOfClass:[JSQMediaMessage class]? )