Session cookie provision

Added classes to:
  - Represent a session cookie (SessionCookie)
  - Provide a current one (SessionCookie::Provider)
  - Obtain a fresh one (SessionCookie::Obtainer)
  - Store it for latter usage (SessionCookie::Storage)

Actually, SessionCookie::Storage hasn't been implemented yet. Nevertheless,
it plays nice with SessionCookie::Provider.

Issue #1.