Touchstone-auth

Updated
- Now works with the new Okta Touchstone flow, for applications that use the Shibboleth proxy.
Added
- The library can now be run as a module (python -m touchstone_auth) for debugging purposes.

Updated
- Library updated to be compatible with the new "universal" Duo prompt, which,
among other things, involves a new `/exit` endpoint to get back to Touchstone.

Updated
- Duo updated to use a second POST instead of a redirect to load some params.
Library updated to be compatible with this new approach

Updated
- Added explicit Kerberos post-auth request to accommodate Touchstone changes.

Updated
- Switched to using a Firefox user-agent, and added XSRF token GET call.

Updated
- Updated the version of requests-pkcs12 we rely on to avoid deprecation warnings.