Brighter, more open theme; footer more like rest of GitHub sites

As intimated in #21 and taking the lead from there, this aims to make learn:github look much more inline with the rest of the sites. Throwing a splash of some Davidoff Cool Water.

Takes a cue (and a chunk of the CSS) from github:training and updates the template to sort of match along with tweaks such as:

- HTML5ify the template
- Pare footer links to most relevant, no need to crowd
- Update copyright year to 2013
- Set code blocks on background more conducive to reading
- Open up H2, H3 heading blocks instead of thick bars
- Header nav in vein of github:developer and github:training
- Header links are calls to action, need spaces between words
- Trim unused CSS rules from templates and Rakefile
- Excise all HRs where possible in favour of CSS

Only checked against Opera 12, Firefox 18, Chrome 23, IE 9, Safari 5.0.5 (Windows)