A lot of improvements to the Sass version

Among a bunch of things:
- normalized comments across all files
- reorganized a couple of things
- added a message system (see _message.scss)
- added a fix for reCAPTCHA (see https://github.com/raphaelgoetter/KNACSS/issues/18)

To do:
- remove _rwd.scss (actually split up across all files)
- keep improving code
- possible turn all lengths to REM with PX fallback
- stop massive commits and do things one by one