Add Bootstrap 3 markup

Since [Bootstrap 3 RC2](http://getbootstrap.com/) is available now, I think it's time to prepare Simple Form to support it.

Changes were introduced in the Bootstrap HTML/CSS API which prevent, for example, fields to be properly highlighted when validation errors occur. I've tried to patch my own code in order to fix temporarily these issues by taking advantage of the LESS syntax. However, despite of some success, the patch is not satisfactory.

```less
// Example of Bootstrap 2.3 to 3 patch
.error {
  .has-errors; // succeed in highlighting labels but not inputs.
}
```


I've never customized much the Simple Form generators, I'll see what I can do; comments are very welcome!