Adding the ability to configure the name of your applications main CSS f...

...ile

Not sure how useful this would be to most people, but I needed to slowly move my styles over to use Bootstrap/a new style guide and wanted an alternate application.css 

Also, it looks like most of the documentation is on the Wiki, so I didn't add anything on this option to the Readme.  Would be happy to add to the Wiki if that is the preferred method of documentation.

Added a configuration option to do so.  Usage:

```ruby
# in config/environments/development.rb
config.style_guide.application_css = 'special.css'
```