Refactor download handling to support generic response content handlers

* Abstracts response download handling so that responses can be automatically converted to an output object, like a `UIImage`, `NSString` or a custom class.

* Custom response content handlers can be created by implementing the protocol `BBHTTPContentHandler` or subclassing one of the implementations on `Handlers/` folder.

* Removes `downloadToStream` and `downloadToFile` properties, converts them in two special response content handlers that have no resulting object.

* If no response content handler is specified, the `setup:execute:error:finally:` helper will assign a `NSData` accumulator by default.

* Adds discarding content handler that discards all contents of response body.