Add ability to include thoughtbot's Bourbon toolset.

This PR adds the ability to utilize thoughtbot's Sass toolset known as Bourbon. The toolset comprises of the following:
- [Bourbon](http://bourbon.io/): a mixin library
- [Neat](http://neat.bourbon.io/): a grid framework
- [Bitters](http://bitters.bourbon.io/): a set of predefined styles

Three additional changes in this PR are as follows:

1. **Set Sass as default choice for preprocessor.** I did this merely because Sass is the more generic/less opinionated option. I suppose setting the default to "None" could be the even less opinionated setup in reality.
2. **Skip Autoprefixer prompt when including Bourbon.** Bourbon itself includes mixins to handle prefixes, so this seemed a bit redundant.
3. **Bumped grunt-contrib-sass to the v0.6.0 release.** I did this because I needed the fix found in this [pull request](https://github.com/gruntjs/grunt-contrib-sass/pull/80).

If anything needs to be changed/reverted for this PR, just let me know. Thanks!