Add support for generating API tokens to User class

[TheFriendlyCoder]

Based on the documentation here it should be possible to use an HTTP basic auth protocol to generate an API token for the currently logged in user so long as the crumb issuer preserves the session that is used to initialize the crumb.

We should add this functionality to our public API, not only for production use but also to facilitate integration of API tokens with the test suite as well to ensure the tests are using a Jenkins configuration that more closely matches the recommended production configuration.