Add support for DataChannels

On the current version of the specification I can only be able to see
references for streams, while I can't be able to see anything related to
DataChannels, the same thing that happened to WebRTC and RTCPeerConnection
object, leading DataChannels to be second-class objects on it. On some
mails on WebRTC mail list it was suggested that if DataChannels would have
been defined before, audio & video streams would have been defined easier
and faster over them thanks to the SCTP hability of having unreliable
connections, and also would have allow to have easier other usages as
chunked audio & video streams (almost) directly on the API, useful for
example for many-to-one P2P streamings.

By the way, I find the decision of using the same API of WebSockets fairly
correct and I don't think it needs to be changed, but maybe I would change
how constructors works making it easier and faster to create a DataChannel
on it's basic usage.