Promise support

[miguelcobain]

Would it be appropriate to change this line in response.js from:

this.source.then(onDone, onDone).catch(onDone);

to

this.source.then(onDone, onDone).then(null, onDone);

The reason for this is that some promise implementations still don't have catch implemented.
A major example is Mongoose/mpromise.
Open to discussion.

[Marsup]

Maybe the catch shouldn't be there in the first place, unless onDone has a chance to fail and can recover with a second call to itself.

[hueniverse]

I have no clue. I don't use promises. I'll let @Marsup make the call on this one.

[Marsup]

@hueniverse Is the last part of my sentence possible?

[bendrucker]

I can't see a way that it would throw such that a second call would resolve it

[Marsup]

Then just removing the catch part should do it.

[bendrucker]

Fix ready to merge in #2324. I don't think it merits a test but I can't certainly add one if desired.