added an alias for new to build to the AR collection proxy, this corrects an issue where the collection proxies were not consistent

Implement proxy_owner, proxy_target and proxy_reflection methods on CollectionProxy with deprecations. Fixes #1148.

- adds tests for find_or_create_by and find_or_initialize_by on has_many associations
- changes the behavior of ActiveRecord::Associations::CollectionProxy#method_missing to differ to
  ActiveRecord::FinderMethods#find_or_instantiator_by_attributes for arg processing and saving so
  find_or_create_by's api on associations will be consistent w/ the api for model classes.

Removing the scope-caching which happens on association proxies, because the query is already cached by the query cacher. For formalised proof see http://www.youtube.com/watch?v=wDefXLb-FDs

Split AssociationProxy into an Association class (and subclasses) which manages the association, and a CollectionProxy class which is *only* a proxy. Singular associations no longer have a proxy. See CHANGELOG for more.