Implementation of reap method to clean up expired sessions (fixes #3)

The implementation works similarly to [the reap method in connect-dynamodb](https://github.com/ca98am79/connect-dynamodb/blob/master/lib/connect-dynamodb.js#L174-#L195).  

A new test was added, tweaked with express-session, and readme file updated.

The default interval is 6 hrs, different than the Dynamo DB but can be set in the options.

We've been running this version of the code and no issues so far.