Implement `Q.dedomrequest`

I'm doing some work with Firefox OS, and many of the Gecko APIs there return a [DOMRequest object](https://developer.mozilla.org/en-US/docs/Web/API/DOMRequest). This seems ripe for promisification a la [`Q.denodeify`](https://github.com/kriskowal/q/wiki/API-Reference#qdenodeifynodefunc-args), especially considering [recent discussion to bridge DOMRequest APIs to DOMFuture](https://groups.google.com/forum/?fromgroups#!topic/mozilla.dev.webapi/q2I0SZzRC-c). Is there a place for this in Q?

Commit message:

> This method method wraps DOMRequest[1]-producing functions and returns
> equivalent versions that returns a promise.
> 
> [1] https://developer.mozilla.org/en-US/docs/Web/API/DOMRequest