Added a layout generator

This is a Rails generator I wrote that makes it easy to include a Twitter Bootstrap template as a layout file. The templates are copied from [the examples][bootstrap_examples] and modified to use the Rails asset pipeline.

**Syntax**
```
rails g bootstrap_sass:layout application --template=fluid
```

**From the USAGE file**
```
Description:
    Generates a layout based on the Twitter Bootstrap example templates.
    <http://twitter.github.com/bootstrap/getting-started.html#examples>
    
    **Note:** The layout doesn't include <%%= yield %>.
    You will need to add that yourself.
    
    The template choices are
      - starter_template (default)
      - hero
      - fluid

Example:
    rails generate bootstrap_sass:layout application --template=fluid

    This will create:
        app/views/layouts/[LAYOUT_NAME].html.erb

```

**Note:** This pull request doesn't include tests. I'm not familiar with TestUnit and I wasn't sure how to write tests for a rails generator.

[bootstrap_examples]: http://twitter.github.com/bootstrap/getting-started.html#examples