Support for ActiveRecord 4.0.x

:fireworks: :fireworks: :fireworks: :fireworks: :fireworks: :fireworks: :fireworks: :fireworks: 

The tests are :green_heart: for mysql, postgresql, and sqlite3. Someone else will need to check the other adapters. 

Since I'm just a :baby: in CPK land, I do encourage you to review my work! In particular, see commit:  https://github.com/codeodor/composite_primary_keys/commit/4c731eff93001f8e19baea7540587f004ddfc211 where I changed the test. Because of changes in Rails which broke some unstated assumptions in the code, I think it was OK to do that. However, I'd like you to review that one in detail to be sure!

I now consider this pull request ready to be merged into the main repo, though if you wish to maintain backward compatibility, you may consider not merging it directly into master, since I made no effort to keep it working with Rails 3.2.x.

The original PR message follows:

:warning: **This is not a complete pull request, and it should not be merged into the master branch!** :warning:

I chose master as the base branch because there is not an "ar_4.0.x" branch in the CPK repository like there is for Rails 3.1 and 3.2.

I'm starting an incomplete pull request because I would not be shocked to learn that a lot of people are already working on compatibility with Rails 4.0, and we are working in isolation instead of having a central body of knowledge.

So far, I've added a few of the obvious cases which I needed to get all of my own app's tests passing. There is a little I've left out, because for the moment they are horrible kludges, but I will fix them and include them too!

After that, I'll begin focusing on the MySQL tests, then Postgres, then whatever other DBs are supported. 

Someone else feel free to join in. I guess fork my ar_4.0.x branch and make pull requests there? Or get with me and I'll assign you collaborator access to commit directly. 

Once we get far enough, we'll update this Pull Request for review by the CPK team.

:+1: ?