Addition of option to allow events to expire after a certain time

Added the ability to specify a value for MongoDb's expireAfterSeconds property.

This can help reduce the volume of data in the database at any one time. From http://docs.mongodb.org/manual/tutorial/expire-data/: "This is ideal for some types of information like machine generated event data, logs, and session information that only need to persist in a database for a limited period of time."

The value is optional and is set in the collector-config file.