Use ClassMethods module
Restructure JoinAssociation module.

- Make #join_association_4_1 & #join_association_3_1 actually private.

- Move the implementation of #join_association to
  ArelHelpers::JoinAssociation.

- Define ArelHelpers.join_association to delegate to the module,
  autoloading it in the process.
Merge pull request #8 from Peeja/refactor-join-association-loading

ArelHelpers.join_association autoloads ArelHelpers::JoinAssociation