Add working mechanism to logout from the site

logout.psp takes care of removing the session variables that make a
session active, hence menu.psp now sends you here instead of sending you
to index.psp.

This fixes #183
Delete the session variables only if they exist

The deletion statements for the session variables, now exist inside a
try-except block so the code doesn't error out in case someone
explicitly tries to go to logout.psp.
Merge pull request #192 from ElDeveloper/issue_183

Add working mechanism to logout from the site