Dynamic authorization by authenticate token

Series of four patches that are the minimum necessary to write multitenant Django-salesforce applications.
It is possible to switch between static and dynamic authentcation and dynamic data server or use both concurrently on two different SFDC connections.

Works correctly also on multithread servers. Database connections related to different organizations or user accounts are safe separated. The same TCP connection can be reused for a different organization/user in the next request, without an additional hanshake.

See the changed README.

(Only 4 commits will remain after applying the PR 78.)