Added ability to join multiple times to AutoQuery, plus test.

I encountered an issue where I wanted to join in two different directions for a single query, and found that it could be supported but just wasn't: http://stackoverflow.com/questions/26783587/servicestack-autoquery-multiple-ijoin

Here is my proposed change to enable this, you are welcome to change the test data to stay in keeping with the theme.

This should be merged into the v4 source tree.
Here I am checking that both Albums and Pets have been retrieved and merged into the DTO.