Conversion of CSS stylesheets to SCSS and leveraging variables

I added a new taks the the `rake` script that makes a SCSS copy of all the CSS stylesheets inside a new `/scss/` directory.

It leverages the "variables" inside the jquery-ui CSS that are used for ThemeRoller and described in this issue:  https://github.com/jquery/download.jqueryui.com/issues/36

So, for easy customization of the jquery-ui CSS you will be able to use the SCSS variant in your Rails app and comfortably customize using variables:

 ```scss
// app/assets/stylesheets/application.css.scss
$borderColorContent: purple;
@import scss/jquery.ui.all
```

(I think some things still need to be fixed for `@import` working properly instead of `//= require`)

So the PR is not 100% done yet, but I wanted to get some feedback early on.

PS:  Once we have this in place it should be fairly easy to include all themes (described in #2) using only some variables.