Remove sauce labs from travis
Update and fix Installation in docs

- Add Bower as an installation methods;
- Fix capitalisation: NPM > npm;
- Remove `.js` suffix from Component.
Remove flashy demos from docs

- Remove section on select demos from Readme.
Update docs for async changes in 27dbfd7

- Update (and refactor) example for Usage;
- Update (and refactor) example for Retext(parser);
- Update description of API: Retext(parser);
- Rename Retext.prototype.parse(source) >
  Retext.prototype.parse(source, callback);
- Update description of API:
  Retext.prototype.parse(source, callback).
Update benchmark results in docs