Add mult classes to markup

Allows multiple classes to be added to markup.

Classes are originally set as a string or array of strings in default bootstrap markup values
e.g.
```
'content-sidebar-wrap' => 'row' ,  
'content' => array( 'span9', 'pull-right' ),
```

This PR does change the parameter passed to the filter __bsg-add-class__ however previously while one could change the class passed an additional class could not be added (so it was broken anyway).

Thanks for @secretstache for helping me work through this. 

Fixes #13