document and detect that session_expire won't work if sessions are stored in memcache

My sense from looking at the code is that this module will not work with sessions stored in memcache, but that fact is not documented on the project page. I think it would be great to document the fact it won't work with that configuration.

It would be even better to have a feature in the module that tries to detect if sessions are stored in memcache and, if so, do a drupal_set_message letting the user know that this module will not be effective in that configuration and the module should be disabled and a new solution found.

Note that an alternative could be for this module to implement hook_init and detect sessions that are over a certain age and expire them. This feature maybe should be optional, but it matches the reason that I am using the module: to give a hard timeout of 10 hours on authenticated sessions regardless of when the user's most recent activity was.