Lots of improvements to API and functionality

* ```$sourceField``` is now ```$sourceFields```, taking an array of fields to search on
* Added ```$sourceSort```
* Added separate ```$displayField``` and ```$storedField``` for the field displayed on selection and the field stored in the DB, respecitvely
* Shortened argument signature, removing parameters  that weren't critical
* Allow chainable set...() methods, consistent with other FormField classes
* Search on each word in the autocomplete separately, to offer more generous results

Example usage:
```php
AutoCompleteField::create(
    'BlogAuthorID',
    'Author'
)
->setSourceClass('Member')
->setSourceFilter("FirstName IS NOT NULL AND Surname IS NOT NULL")
->setSourceFields(array('FirstName','Surname','Email','Nickname'))
->setDisplayField('Name');
```