update pods
update default map region span to 500 meters
add new JSQMessagesMediaViewBubbleImageMasker. ref #571
add convenience method to JSQMessagesMediaViewBubbleImageMasker. #571
mask media views for JSQPhotoMediaItem. ref #571
new abstract base class, JSQMediaItem, for pulling out common media item behavior. refactor JSQPhotoMediaItem to inherit from JSQMediaItem. #571
change JSQLocationMediaItem to inherit from JSQMediaItem. mask JSQLocationMediaItem. #571
change JSQVideoMediaItem to inherit from JSQMediaItem. mask JSQVideoMediaItem. closes #571.
refine category unit tests. update image category for asset methods.
more unit test refinements
update umbrella header
update version numbers
Merge remote-tracking branch 'upstream/develop' into develop

Conflicts:
	JSQMessages.xcodeproj/project.pbxproj
	JSQMessagesViewController/Model/JSQPhotoMediaItem.m
	README.md