Make a config option for session timeout duration

[volomike]

In the bootstrap/front controller (aka "index.php"), I'm hard-coding the session timeouts to one hour. However, people may want longer session timeouts, such as a guy who uses session variables for shopping cart items that he wants to cache for an entire day. So, I'm going to change this to use an app/config.php option like SESSION_TIMEOUT_MINUTES and set it to 60 (for 1 hour) by default.