Integrate bootstrap CSS

The main changes are :
- Swapped the icons for the twitter icon set.
- Used bootstrap buttons.
- Re-jigged the forms to fit in with the bootstrap CSS form classes.
- Restyled the devise views (bootstrap form) and added a devise layout.
- Split the CSS into screen.css (the bare minimum to do layout) and skins/default.css (styling/coloring only).

Some of the bootstrap stuff mixed in with the jscript might not be terribly semantic - it was a tossup between using bootstrap classes throughout, or using mixins in the CSS, which would have meant a lot of duplication in the compiled CSS and a pretty big size boost. 

The logo is from the twitter icon set - it's actually taken from the paid version of the icon set, which has larger sizes of the free icons, and I colored it myself, but is otherwise identical to the smaller free icon. The license doesn't forbid distribution as part of a project as far as I can tell as long as they're not 'resold' - may be worth getting clarification on this or removing it if you think it's dubious. All the other graphics are part of the free icon set distributed with bootstrap, with a bit of coloring added.