Refactor JSQMessagesBubbleImageFactory to enable custom images/insets

The factory is now instantiated, rather than used as a set of static methods.
The default init function creates a factory which behaves exactly like the previous version.
The two-parameter init function takes a custom image and user-defined strechable regions.

Two new unit tests cover the custom image constructor
remove unexpected whitespace
Import of 'JSQMessagesInputToolbar' instead of forward class declaration for 'JSQMessagesViewController'.
Merge pull request #537 from AnthonyMDev/develop

Import of 'JSQMessagesInputToolbar' instead of forward class declaration
fix imports per #537
re-add stroked bubble assets. close #534
add bubble image asset helper methods to category. #534
Merge pull request #535 from walsh2000/develop

Refactor JSQMessageBubbleImageFactory to allow custom images
follow up from PR #535