Make all requests cancellable

[Arech]

Hi!

It would be very nice, if the library will provide some means to cancel requests. Exponential back-off algo is really cool, but sometime it is absolutely necessary to be able to cancel too long running request.

[rjrodger]

That is an excellent idea! I'll need to think about a good API for that - a callback that returns true to stop the request?

[Arech]

I'm not sure if a callback will be good... I think it is better to use traditional handles, like a work with timeouts:

var h=setTimeout(..);
...
clearTimeout(h);

[rjrodger]

Arech adding you as a contrib, see https://github.com/rvagg/node-levelup/blob/master/CONTRIBUTING.md