Add helper function to serialize list of structures

For query services that have operations that accept a list
of complex types, this helper function provides a simple
way to serialize those params.  There are several services
that can be refactored to use this new method, but I think
it makes sense to do that on a case by case basis (and as
separate commits).

I added unit tests for this as well, but given that these
were for the AWSQueryConnection class, I put the test file
in the top level tests/unit directory.
Remove erroneous comment in build_list_params

I believe this was from a commit that eventually was
reverted so the comment no longer applies.
Merge branch 'complex-list-params' into develop

* complex-list-params:
  Remove erroneous comment in build_list_params
  Add helper function to serialize list of structures