Allow string class name to be passed in to #relationship helper

This allows the syntax:

```ruby
  has_n(:products).to(Product).relationship('Order::Line')
```

Which is important especially for namespaced relationships.  Otherwise, UninitializedConstant errors would be thrown.

Note that there were more changes to the test suite (which passes now with +1 test) than to the acutal source.

 - There is an additional test for this case.
 - How classes are asserted has changed.  Before, an instance of a class with to_s implemented would be compared.  This caused issues with the class instances being compared but not equated (`       Diff:Class.==(Class) returned false even though the diff between Class and Class is empty. Check the implementation of Class.==.`)  This change simplifies and reads better, so I figured it was a good way to go.