Refactoring CSS Pre-Processing

Pull request for [this issue](https://github.com/yeoman/generator-webapp/issues/237) consists of 2 major changes:

1) Binding Compass functions to the compassBootstrap (now includeCompass) variable. As mentioned in my issue, there was another relatively popular issue on [making the Compass grunt task optional](https://github.com/yeoman/generator-webapp/issues/114). Compass functions are now only included if the user selects to use Sass pre-processing.

2) Separate Bootstrap from Sass with Compass. This was a major change discussed between @addyosmani, @sindresorhus and myself. Rather than the previous `Bootstrap with Sass` option, the prompt is now:

```
[] Sass with Compass
[] Bootstrap
[] Modernizr
```

which results in the following 4 workflows:

* Just CSS
* Just Sass
* Bootstrap with CSS
* Bootstrap with Sass

Each workflow has been tested with `grunt` and `grunt serve` functions.