Add foreign key support

[icedtoast]

Based on #17 this allows foreign keys and hierarchical uris.

Add support for foreign keys on android versions which support it, as well as adding support for heirarchical urls so that you can have paths like: /posts/1/comments which refers to all the comments for the post with id 1.

[bhurling]

This is a great change. I already thought about a feature like this. At first sight your changes seem reasonable. But I need some more time to test it. Thank you very much for your contributions.