Create Sass themes from existing CSS

I extracted a default theme from the existing CSS called uchiwa-default. I also added a dark theme (palourde/uchiwa#18) called uchiwa-dark. The dark theme is a good example of how to create additional themes.

To switch to the dark theme remove uchiwa-default from header.html and add this in its place...
```html
<link href="css/uchiwa-dark/uchiwa-dark.css" rel="stylesheet">
```

The generated CSS theme files (uchiwa-default.css and uchiwa-dark.css) should not be edited directly. They should always be regenerated from the Sass files, like so...
```
cd uchiwa/public/css/uchiwa-default
sass uchiwa-default.scss > uchiwa-default.css
```
and
```
cd uchiwa/public/css/uchiwa-dark
sass uchiwa-dark.scss > uchiwa-dark.css
```
Many modern IDEs have file watchers that automatically handle this process. [Webstorm](http://www.jetbrains.com/webstorm/webhelp/using-file-watchers.html), for example, is one such IDE.