3.0: Change the searchable behaviour to use finders

This is a rather major change so that the searchable behaviour makes use of finder methods instead of arbitrary table/model methods.

Instead of calling `Table::parseQuery($data)`,  you would now do `Table::find('searchable', $data)`.

Likewise, the query, subquery and expression types have been replaced by a single finder type:
```
'field' => [
   'type' => 'finder',
   'finder' => 'byTags'
]
```
The new CakePHP ORM makes it trivial to perform the functions that the three previous query types allowed.

In addition, the documentation has been updated to reflect the changes.