Support for :primary_key option on the source reflection of a through association, where the source is a has_one or has_many

Refactoring: replace the mix of variables like @finder_sql, @counter_sql, etc with just a single scope hash (created on initialization of the proxy). This is now used consistently across all associations. Therefore, all you have to do to ensure finding/counting etc is done correctly is implement the scope correctly.

Add assertions for nested through associations loaded by includes with conditions (uses the single-query strategy). Currently one failure to fix.

Conflicts:
	activerecord/lib/active_record/associations.rb
	activerecord/test/cases/associations/cascaded_eager_loading_test.rb

Added foo:install:assets task that copies assets from plugins public directory to application's public directory

This is the most simple and naive approach: just copy every files from engine to app.
The only exception is when file has changed, in that case developer will be
asked if he wants to rename the field.

There is no need to make this task more sophisticated as 3.1 will be shipped
with better assets handling and it will be the default way to handle things.

This reverts commit 9c53e82d.

	This makes a test fail, but it is revealing a bug in Arel master.