Support Netty 4

[slandelle]

WIP done in netty4-support branch.

Doc: http://netty.io/wiki/new-and-noteworthy.html

Had to start from scratch because both Netty provider and Netty had changed a lot since first PR.

TODO:

• find a way to handle HttpContent ref counts/allocation. My 2 cents:
  • most users would not want to manually handle allocation (would have to manually free chunks in every case: onComplete and onThrowable), so we'd have to quickly extract bytes and free chunks
  • some, like myself would like to in order to save many byte array copies

[slandelle]

Done in master.