Prompt for CSS Preprocessor

Closes #8

- Add cssFramework list prompt; default to LESS
- Update Gruntfile to use sass or less task
- Remove less from package.json, push it in based on choice
- Set compiler option in middleware
- Add .sass-cache to gitignore
- Update comment about requestStart run time
- Rename app.less to _app.style in case we want custom content
- Update app tests to reflect changes and add SASS specific one

Depends on paypal/kraken-devtools#5 being merged in order to work correctly unless we want to install both sass and less dependencies.