Added bottom padding to all bootstrap themes

As discussed in issue #251 I added padding to the main document area to all bootstrap themes.

I have updated the Base.css to have 100px padding, like this:
```
div.col-md-9 {
    padding-bottom: 100px;
}
```

Normally, I would choose to do it differently - like to assign a specific class to the sidebar and main area, but I followed the convention in the CSS, since I saw there was already this:
```
div.col-md-3 {
    padding-left: 0;
}
```

Screenshots of the Yeti theme, before (top) and after:
![beforeafter](https://cloud.githubusercontent.com/assets/2405099/5235874/c99359de-7820-11e4-93c4-490dcd4f550a.png)