Added a prefix method.

$.tokeninput will now expect an object for the prefix method.

I added the functionality for a prefix method. Now a user can put a symbol before a word and that word will get auto filled. It works like this:
```
$("#baller-token-input").tokenInput({
                                     prefix : true,
                                        '@' : ['/person', {freetagging : false}],
                                         '#' : ['/tag', {freetagging : true}]
                                    });
```