First step toward a session-auth plugin.

(See #1, and cburschka/cadence#31)
Adding a README.md to session/
Formatting of README headings.
Implement smf2_session and finish RPC code. (#1)

This patch should be ready for testing with SMF.

(cburschka/cadence/#31)
Finished session auth framework for now. (fixes #1)

Doc changes and some implementations remain.