[#572] Improving the percolation support in Tire::Model::Search and Tire::Model::Persistence

* Added integration tests for search and persistence
* Removed class variable which was cross-pollinating the class object space

Related #572
[#572] Do not use `@attributes` in Tire::Model::Search percolator integration

The `@attributes` Hash in Tire::Model::Search clashes with ActiveRecord namespace.

The Hash has been renamed to `@tire__attributes`

Closes #572