Added base tag, made urls relative so the docs can be hosted from a subd...

I would like to host the docs from http://localhost/~robkorv/bootstrap.
With the base tag you can host it from anywhere.
Make the base tag absolute and al the href tags relative.

```html
<base href="http://localhost/~robkorv/bootstrap/">
```
Will link
```html
<a href="components">Components</a>
```
To http://localhost/~robkorv/bootstrap/components
Use 
```html
<base href="/">
```
To host everything from the root, like it is now.