Move something in JoinDependency and JoinAssociation from squeel to polyamorous

Hi @ernie,

I have moved code which combined Polyamorous tightly in Squeel here. 

Includes:
1. All contents in `squeel/adapters/active_record/4.1(4.2)/join_association_extensions.rb`
2. `build_with_squeel` method in `squeel/adapters/active_record/4.1(4.2)/join_dependency_extensions.rb`

`walk_tree` uses classes of `Squeel::Nodes` , so I think it shouldn't be moved here.
I'm not sure about `join_constraints` method.

Would you please have a look? If there are more things need to be moved, just tell me. After this patch is merged, I will remove relative code in Squeel immediately. Thank you very much.