Only show messages for now (not embeds etc)
Cache scaled avatar images in NSCache

Meanwhile caching original images in LFSAuthor objects.
Additionally modify behavior of LFSAuthorCollection such that
new objects being added with the same id as an existing object
are not added as separate ones but simply act to update the data
behind the existing object.
Merge pull request #34 from Livefyre/dev

Cache scaled avatar images in NSCache