Refactor to better capture the semantics of LFSResource

LFSResource is the result of merging LFSHeader and LFSTriple. It is a
simple convenience class that encodes URL-like information except
together with associated image, except that the identifier field
does not have to be *universal* (it could be, e.g. a Twitter handle).
Merge pull request #62 from Livefyre/dev

Refactor to better capture the semantics of LFSResource