[jove] Add new extractor. Closes #3177

Hi,
this adds support for [jove.com](http://jove.com/), as requested in issue #3177.

Supports videos with *Open access*. Videos that require a subscription (http://www.jove.com/video/51796/, for example) are also extracted, but they contain only the first 20 seconds of video, the rest of the video is replaced by the Jove logo (audio is intact).

Suggestions / improvements are very welcome.