Aggregator contribution (release 2.2)

As communicated before, I'm submitting the aggregator pattern to akka-contrib (and wish it will be included in mainstream). Please consider adding it for future releases. BTW, if the MIT license header is against the rules, I'm happy to remove it. Any concerns, please email me directly at akara.sucharitakul@gmail.com. I'm definitely flexible if further guidelines need to be followed.

Thanks!
-Akara