Add content model

Content collection for now is a subclass of NSMutableArray (TODO: implement a ring buffer?)
Author collection meanwhile is a subclass of NSMutableDictionary.

Both normalize objects added to them (will accept either objects of type LFSAuthor/LFSContent
or respective JSON objects).

LFSContent has a flattened representation (i.e. metadata is flattened to the same level as content).
Get closer to true ORM

Instead of passing around both author and content, have author to
be a property of content.
At last: correctly generate remote/Twitter URLs

For pieces of content that come from Twitter, generate “view on Twitter” urls.
Make remote Twitter links conditional

Only show these links when the content actually has them
(i.e. comes from Twitter).
Merge pull request #9 from Livefyre/dev

Dev