Mongoid4 fixes - Don't include Mongoid::Matchers (we don't need them) and they cause problem with mongoid4. Add condition code to clear identity map in spec runs in mongoid2 and mongoid3 only. Fix photo model to have comment field (otherwise mongoid4 complains).
Add ruby 1.9.2 back to test matrix but exclude mongoid3. Also run mongoid3 and mongoid4 on ruby 1.9.3.
Update readme to make it clear this fork of streama uses Fan Out On Write, unlike christospappas's version of Streama, which uses Fan Out On Read.