NSFW image detector plugin

This pull requests add support for a new "NSFW image detector" plugin. This is mostly a foundation to built upon since the first iteration of plugin is very basic and does simple detection of nudity based on the skin tones in the image.

Besides that, I also made some other changes:

* Cleaned up some pep8 violations
* Removed trailing whitespace
* Started to bootstrap a slightly better plugin architecture

Hope you don't mind, I just couldn't resist cleaning up the code :P

If you want, I can create a separate branch for style commits to make a diff smaller and more bearable or remove style commits altogether.

I'm also not 100% sure about including potentially "offensive" test fixtures in this repository. Current version doesn't contain any actual nudity so it should probably be fine though.