Investigate canceling PouchDB ajax requests when the context is cancelled

[flimzy]

At the moment, the only explicit Context support in the PouchDB bindings is for timeout contexts, which sets the AJAX timeout. (And, of course, just returning early, while leaving the process running in the background, if a context is cancelled--not ideal). But cancelling contexts is probably more common, and I should try to cancel an existing AJAX connection when that happens.

[flimzy]

Issue moved to new repo: #666