Add RSpec support

Not ready for merge. Adds configuration to allow the helpers to be used in RSpec. Separates into a few files:

```
lib/challah/test/helpers  # The helper methods
lib/challah/test/unit     # Test::Unit integrations
lib/challah/test/rspec    # RSpec integrations
```

One thing that stands out as odd to me is [this bit](https://github.com/philtr/challah/blob/1bc3deb5001ce5aa1c39258a881b1b8e02e302ae/lib/challah/test.rb#L4-L6):

```ruby
if defined?(RSpec)
  require 'challah/test/rspec'
end
```

But alas, this is how thoughtbot [does it with shoulda](https://github.com/thoughtbot/shoulda-matchers/blob/master/lib/shoulda/matchers.rb#L4-L6). If you can think of a better way I'm open to ideas.

By the way, all the current tests pass, but I am at a bit of a loss as for where to start testing the RSpec stuff. Maybe I will look at `shoulda-matchers` and see how they do it.

I'd love to hear your feedback on this feature.