Update CloudFormation to support the latest API.

This pull request includes the following:

* Stack create/update supports new parameters
* Refactoring of common code for JSON requests
* Documentation updates
* Added `estimate_template_cost` method
* Added `get_stack_policy` method
* Added `set_stack_policy` method
* `Stack` objects now have `get_policy` and `set_policy` methods
* Added unit tests for new methods
* Added integration tests for more methods

Along with 1dfecb0a40c093ec3bd4eabae0bd1f0262272d14 this now means the tests are run with the standard test suite.

@toastdriven, @jamesls please review.