[completed] The logout operation.

using the built in PHP session to distinguish the session data used by CI
and by us.

The built in session is used for only one thing. The login and logout check.

Everyting else is done using the CI sessions.
[removed] The echo statement in the header.
[removed] The echo statement for echoing the passwords in the checklogin function of the userdb model.
Merge pull request #10 from icyflame/logout-feature

Added the logout operation