add mediaconstraints to createoffer

So, I was playing around with offerToReceiveAudio as Ben suggested.

It does indeed improve what we see on `chrome://webrtc-internals/`: `Conn-audio-1-0` is replaced with `Conn-data-1-0`. No functional difference that I can see but it does seem to be cleaner, and the SDP offers and answers are significantly shorter, too.

It seems you need to supply the constraints to `createOffer` and `createAnswer` which is a bit annoying...not high priority but curious what you think of this approach.
The latest draft actually doesn't support this argument on createAnswer.  Are you sure it's needed?