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
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
clean up
handle iOS 7 and 8 specific issues. close #5221. close #544. ref #484.
better handle layout invalidation. close #549
beta 3
remove old comment
pod update
update JSQSystemSoundPlayer. beta4
update bubble assets for @3x close #524
add play icon for #504
fix bubble assets #524
Updated README to reflect beta4
Merge pull request #551 from ranunez/develop

Updated README to reflect beta4