RSpec 3.0 Support

Perhaps I'm raising this a bit too early, but now that RSpec 3.0 is in beta1 I thought it might be a good time to raise an issue in shoulda-matchers.

The upgrade process has been painless on my end with [yujinakayama/transpec](https://github.com/yujinakayama/transpec) and a bit of awk + grep.

Tests are green but there are a lot of deprecations in the matchers:

```
Shoulda::Matchers::ActiveRecord::AssociationMatcher implements a legacy RSpec matcher
protocol. For the current protocol you should expose the failure messages
via the `failure_message` and `failure_message_when_negated` methods.
```

If this is too early to bring this up feel free to let me burn in :fire: