Added a "sample_two_factor" authentication, demonstrating support for multiple methods.

Refactored context files, separating settings per authn method as much as possible.
Added a "sample_two_factor" authentication, demonstrating support for multiple methods.

Refactored context files, separating settings per authn method as much as possible.
fixes JavaDoc to reflect createCredentials() no longer returns null
stop asserting null where exception thrown in lieu of null
Noted where argument extractor configured.
strong_two_factor state id now uses underscores rather than dashes
Document `sample_two_factor` authentication method example.
Merge pull request #24 from Unicon/2ND-SAMPLE-AUTHN-METHOD

Adds a second example required authentication method, `sample_two_factor`.
Demonstrating webflow-layer support for multiple authentication methods.