Tweak homepage styles

- add clearfix to h2 elements so .hint is properly contained; otherwise the code area becomes too narrow
- switch to floats for two-column layout making it possible to add things below the code (I just added a 40px margin)
- instead of a min-width at which the two columns would have no padding, I switched it to have constant 40px padding in between the columns and to divide the remaining space equally

Before: https://f.cloud.github.com/assets/6820/1386353/9246758a-3b70-11e3-998b-d69e4ea8d8de.png
After: https://f.cloud.github.com/assets/6820/1386356/9a4be97c-3b70-11e3-9265-072f78c06ac0.png
Merge pull request #24 from spicyj/gh-pages-col-styles

Tweak homepage styles