Another API issue: addStream has no callbacks

Sometimes it seems like I'm screaming into the void by posting the 
issues we're actually running into implementing this stuff, but here's 
another problem we've uncovered with the currently defined API:

addStream() can plausibly fail, and some of those failures can occur 
asynchronously. Examples of such failures include exhausting encoders of 
the indicated type (e.g., I might have a device with only one hardware 
video encoder).

While we could allow these calls to succeed and then return an error 
when the application attempts to take further action (e.g., createOffer 
or createAnswer), I think delaying such errors until later will surprise 
and befuddle application developers.

For these reasons, I would propose the addition of success and error 
callbacks to addStream.

/a