Re-do documentation and README (successor to #3094)

At the moment, `youtube-dl`'s documentation is a bit of a hodgepodge. Some of it is done with Sphinx, some with odd python scripts. The `README.md` is not so much a README as a manual, and it is not very useful for someone trying to get a quick overview of the project. This is my attempt to fix this, using [ronn](http://rtomayko.github.io/ronn/). See also #3094.