Find_with_case improvements, bump to 3.3.0 

### WHAT

* Improve `find_with_case` lookups (closer match to Rails)
* Increase `ActiveRecord` dependency from `3.0` -> `3.2` 
* Remove old methods in `ActiveRecordTrickery`
* Bump gem version to `3.3.0`

### WHY

I ran into the `_`/`LIKE` collision with the previous implementation of case-insensitive `find_with_case` lookup, but then realized that it was changed from `LIKE` to `LOWER` recently in a889f9a3f3611ece7a764edc02fb86e83e32d208 . 

But most string columns in MySQL are case-insensitive, so why not let `ActiveRecord`'s adapters do the work for us and use a simple `=` query if the column is already case-insensitive.

In addition, the documentation states that this lookup method was modeled after the uniqueness validator in Rails, so why not clone its implementation. The way I've implemented it is basically the same as in the `UniquenessValidator` of Rails 3.2 and 4.0 (https://github.com/rails/rails/blob/3-2-stable/activerecord/lib/active_record/validations/uniqueness.rb#L55).

I think we should also increase the dependency on `ActiveRecord` from 3.0 to 3.2 for the sake of our sanity.