Find Mongoid associations

As noted in the extended commit message, there was previously asymmetry between creating and finding models when using Mongoid:

Given a Post model that belongs to a User, a Given step like this would work:
    
    Given the following posts exist:
      | title | user        |
      | foo   | user: "bob" |

but a similar Then step would not:

    Then the following posts should exist:
      | title | user        |
      | foo   | user: "bob" |

The functionality of loading associations in `convert_models_to_attributes` doesn't really seem to be covered at all in the 0.4.x specs -- sorry for copping out and not adding some to cover it! It seems like something that would be best handled for various adapters with integration tests, and upon thinking this I saw your 0.5-unstable branch where it looks like Cucumber steps may be in the works for just this sort of thing. If I have a chance I'll have a further look at that branch, and take a stab at verifying this if it's not covered already.