Add denormalized structure builder

Useful for writing tests or manually creating structures.  It's
typically easier to hand write a denormalized model than it is to
write a normalized model, at least for simple cases (as is common
in tests).

cc @danielgtaylor @kyleknap 
Is there a reason why this test is not a method of a class that subclasses from unittest.TestCase? I feel like it is not consistent in that way with the rest of the tests in botocore.