boto cloudformation : Add StackResourceDetail object

Encapsulate DescribeStackResource responses in a new StackResourceDetail object, meaning only the
Metadata needs to be parsed as JSON not the whole response.  

This aligns the boto interface for DescribeStackResource with the other API calls and makes it more intuitive wrt the AWS documentation.

Note this has only been tested with the heat (heat-api.org) AWS Cloudformation API implementation as I don't have access to test with Amazon's CFN API.