Add Account client with VerifyCredentials command
Add instructions for running tests to README
Allow marshalling responses into "responseClass"

This is documented to work already with Guzzle 3.2.0, but it doesn't.
Basically, you can put a "responseClass" field in an operation in the
service description, and responses will be put into that class.
Use ModelFactory to create response class objects
Improve GuzzleTestCase::setMockResponse function

Implemented a convention for the mock response directory structure,
allowing the response files to be automatically found if they're
arranged correctly.
Add basic CaseClient and GetCases command/iterator

Needs more validation around the GetCases command parameters, and then
add more commands.