Move UltiSnips snippets into vim-snippets.

Please only review and do not merge - I want to orchestrate the deletion of snippets from UltiSnips, adding snippets here and releasing UltiSnips 3.0. I think I have push rights to the repo.

This pr moves the current snippets from UltiSnips into vim-snippets. When this is merged all UltiSnips snippets are curated in this repository and I take on maintenance of the UltiSnips/* files. 

This PR also cleans up the README and removes the outdated and neglected AUTHORs file. See inline comments for my reasoning for changes in README - these are the only thing that need review really.