Dynamic scopes and dynamic finders for belongs_to

This patch consists of two changes:

1. the dynamic finders now accept belongs_to attributes.

2. attributes that can be used via a dynamic finder can also be directly accessed as scopes.

Example usages include:

  comments = Comment.find_by_creator(current_user)

  comments = Comment.creator(current_user).commentable(post).all

  posts = Post.creator(current_user).published.my_named_scope.last

  cases = Case.status("OPEN").priority("HIGH").my_named_scope.all

The main advantage of this patch is that it allows easy formation of dynamic queries based on input, without needing to explicitly define named scoped in your models. For example:

  scope = Comment.scoped({:include => [:creator, :commentable]})
  scope = scope.commentable(post) if post
  scope = scope.university(params[:uni]) if params[:uni]
  scope = scope.creator(current_user) if params[:created_by_me] == "1"
  scope = scope.country(params[:country]) if params[:country]
  @comments = scope.all