Add scrolling, multiget, facet ordering, type checking, features and bug fixes

[erickt]

Heya karmi!

I finally had some free time to write some bug tests and do some cleanup, so I closed my old pull request #352, and opened this pull request. The big changes are now almost everything has a test now, and the Model#save will no longer throw an exception if there's a conflict error. It does return a False though. I also factored out the multiget api for a future pull request.

[kofno]

This is a big pull request. It mentioned multiget in the title, but I didn't see it in the impl.

[ches]

Bump on the notion of breaking this up and cherry-picking more manageable bits, like Persistence bang methods.