Files cleanup

1. Re-enabled `xml` NIF. Will test soon if it changes performance somehow.
2. Removed old, unused files, scripts and documents
  * Documentation has moved to GitHub wiki, so `doc` folder is left as a placeholder of images for documentation.
  * I think it was safe to remove MS SQL files as most probably they are outdated and we don't support MS SQL yet anyway.
  * @studzien added `src` in the root of the project with some explanation but project compiles just fine without it. Does anyone know why it was added back then?

Please let me know if someone thinks I removed too much.