Fixed issue https://github.com/TheRealCharDev/mongoose-session/issues/1

Added and `options` second parameter
Added `options.ttl` to set the session expiration delay
Added automatic callbacks if not set (before it crashed without any callback provided)
Updated express-session in dependencies
Updated README
Removed mongoose in dependencies
Merge pull request #2 from Lesterpig/master

Fixed issue https://github.com/TheRealCharDev/mongoose-session/issues/1