Upgrated Gems to pre-release Susy-Next, Sass, Compass

Also adding Breakpoint as a dependency to work with Susy 2.

Can't use the latest Jeyll (1.4) because the compass and Jekyll 'listen'
dependencies are incompatible. Hopefully this will get resolved.
Add rbenv rehash instruction
Update layout to Susy-Next
Split sass into its own Gemfile in assets

- Update jekyll gems, include rdiscount
- Make sure to ignore vendor files from assets for jekyll
- Update README with dual Gemfile setup
- Ignore assets/vendor