[#10] Added LastEventId argument to init() callback. Added {no_content, Req} result for init.
[#10] Added test cases for last-event-id and 204 No content functionalities.
[Closes #10] Updated README.
[Closes #10] Updated init callback definition with additional possible return value.
[#10] Removed last_event_id() and used cowboy_req:header().
[#10] Replaced the use of cowboy_req:headers/1 for header/2.
Merge pull request #12 from inaka/jfacorro.10.missing.rfc.features

[#10] Implemented missing features form the RFC