providing ability to manage flat_query on a filter class basis

[ssteffen]

This makes flat_query more easily accessible by doing the following:

• Moves the FlatQueryTools module out of the Filter class. Instead, the implementing filter class should include it - and then flat_query will be enabled by default
• Creates a smart default method on filter called can_use_flat_query?. Can be extended to handle custom use cases
• Changes the standard get_query method to check can_use_flat_query? method and whether the FlatQueryTools module is included. If so, it uses flat_query. Otherwise it calls get_complex_query which is simply the sub-select path which is the standard default