Imgur module improvements

Some changes to the imgur module to make it more robust.

* Now handles various errors, like non-200 status responses and invalid json.
* Added an includeNSFW member var that can be changed to allow or dissalow nswf. Might want to to be an option somewhere else.

Made this a PR because it was some quite major changes. Also can't test the changes for "real" at the moment (curse you proxies!).