Converts data with content-type starting by "image/" to UIImage.
Converts data with content-type equal to "application/json" to json representation.

This will construct a new NSError object with domain SVHTTPRequest and
a code matching the HTTP request which is passed as the error to the
completion block for all non-2XX status codes.

Requests can now be cancelled via [request cancel] which
will return YES if the request was successfully cancelled
or NO if the request was already finished.

Completion blocks will be called with a response of nil
and an NSError with SVHTTPRequestCancelErrorDomain.

…kground threads (bbdef7d).

- Increase delay before timing out
- Cancel requests after the timeout delay