Fix failing unit tests, added optional server cert validation

This should probably have been two separate pull requests, sorry, but they're both small and separate.  I can branch and resubmit if needed.

Cert validation:

Server certs are validated if optional CA bundle file is given.  Added to README and used in the tests if specified.

Uses the cert-validating https_wrapper, Apache 2.0 from the Google App Engine SDK:
https://googleappengine.googlecode.com/svn-history/r136/trunk/python/google/appengine/tools/https_wrapper.py

Unit tests:

Unit tests were failing; some account elements in fixtures didn't have an account_code element. Added them, tests pass.

I don't know if this is the right thing to do, but every other account element in that fixture subdir has the account_code element.