Rename LFSMutableContentCollection -> LFSContentCollection

This object is quite useless without being mutable so we adopt NSMutableDictionary interface and rid of Mutable in the name because of redundancy (the class no longer has an immutable equivalent).
Auto-generate certain lazy instantiation getters in model classes
more NSNull-aware automatic wrapping

Since the raison d'être of NSNull is to allow one to represent nil values in NSDictionary or NSArray, it makes sense to translate NSNull to nil as soon as we wrap the corresponding parent object.
test for truthy result object when liking/flagging
update README (not so basic anymore)
Merge pull request #77 from Livefyre/dev

Auto-generate some getters in model objects