Deprecated options `delete_sql`, `insert_sql`, `finder_sql` and `counter_sql`
have been deleted.

Added some more documentation for define_readers and define_writer of the Association and its inherited classes

Suggested by @dhh.

It doesn't affect the generated SQL, so seems reasonable to continue to
allow it as an association option.

Method compilation provides better performance and I think the code
comes out cleaner as well.

A knock on effect is that methods that get redefined produce warnings. I
think this is a good thing. I had to deal with a bunch of warnings
coming from our tests, though.

Well, not all of them, but some of them.

I don't think there's much reason for these methods to be private.

This reverts commit 3803fcce.

Conflicts:
	activerecord/CHANGELOG.md

It will be deprecated only in 4.0, and removed properly in 4.1.

This change uses Module.redefine_method as defined in ActiveSupport.
Making Module.define_method public would be as clean in the code, and
would also emit warnings when redefining an association. That is pretty
messy given current tests, so I'm leaving it for someone else to decide
what approach is better.

Instead of generating association methods directly in the model
class, they are generated in an anonymous module which
is then included in the model class. There is one such module
for each association. The only subtlety is that the
generated_attributes_methods module (from ActiveModel) must
be forced to be included before association methods are created
so that attribute methods will not shadow association methods.

Use proper objects to do the work to build the associations (adding methods, callbacks etc) rather than calling a whole bunch of methods with rather long names.