New RSpec DSL

Support writing specs like this:

```ruby
describe '#name' do
  property(:user) { User.new(first_name: string, last_name: string) }

  it 'concats #first_name and #last_name together' do
    user.check { |u| u.name.should eq "#{user.first_name} #{user.last_name}" }
  end
end
```

I typically avoid overriding method missing but this seemed like a really good usecase for it and since we're just checking if the `test_data` responds to the method name or not it should still be pretty performant.

This is the first step in trying to integrate Rantly more deeply with RSpec and Minitest... Would love to get rid of the `puts` statements and instead include that information as part of RSpec (or minitests) normal output... Working slowly towards that.