Refactor to make sure get_subclass() is on QuerySet

[tswicegood]

There's an edge case where you might want to call get_subclass() on a
QuerySet the same way you can call get() after you have already called
various filter/exclude methods.

[carljm]

Thanks Travis! Any chance you'd be willing to add a test for using get_subclass on a QuerySet?

[tswicegood]

Not sure when I'll be able to get to it. I'm assuming there's a few tests for get_class so I could just copy those. I did verify that the py2.7-1.5 environment does still pass.

[tswicegood]

@carljm Added a test that verifies that all().get_subclass() chained works. Not a direct test on the QuerySet, but does fail against master.

[carljm]

Looks great, thanks!