callbacks.{call,apply,bind,promisify}

(final?) part of #73.

I've implemented the functions as we discussed on the above issue, including the object-based API for `promisify`. I'm not fully satisfied by the implementation (maybe there's a way to make it clearer that I'm not seeing?), but I think it should be enough for a conversation starter.

One thing that we haven't directly discussed is whether `promisify` should have the ability to partially apply arguments, as `bind` does. I haven't included that ability for now, but should be easy to do.

Also, on a slightly less technical issue, I have no idea of how to write the header of the file (the one with the licensing and all that) :) Should I just copy it from one of the other files?