Rubber tasks for creating ElastiCache Clusters

I've decided to play around a bit with ElastiCache.  After a lot of confusion, it seemed best to put this in the artifacts sections of the instance-*.yml.  This would probably be a good place for other AWS services such as RDS as well.  

I haven't gone about having this override the memcached or redis roles yet, which I'm thinking about doing as a standalone template for elasticache.

Added:
* cap rubber:elasticache:create
* cap rubber:elasticache:destroy
* cap rubber:elasticache:describe

Saving to the artifacts part of instance yml.
Next up is to write overrides for memcached.yml and redis.yml

Aside:
We need some better documentation for the internals of Rubber.  It's tough to find your way around at first and there's not enough testing to show you the way.