ENH: add a new method to create and authenticate a session in one go.

This can be useful for simple utilities who do not need to be robust
aginst invalid/incomplete configurations.
DOC: streamline imports in doc examples.
Merge pull request #419 from enthought/add_authenticated_config_api

Add authenticated config api
ENH: allow creating filled-up configurations at construction time.
DOC: update example to use new ctor feature in Configuration.

[skip ci]
Merge pull request #420 from enthought/allow_update_in_init

ENH: allow creating filled-up configurations at construction time.