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Likewise, the query, subquery and expression types have been replaced by a single finder type: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.
require (not required)