Added strict serializer only supporting gif

Standard AFNetworking serializer not only assigns image, but also checks
for status codes, MIME types, and inflated images on background thread
before assigning. Loosing this behaviour on all images is unwanted,
thus another, more strict serializer is created to be used with compound
serializer (see example project).
Added serializer usage to README.md
Merge pull request #51 from coverback/improved_serializer

Added strict serializer only supporting gif