knife-aws-base dependency

Depending on knife-ec2 for common options parsing seemed a bit heavy. I've refactored the common parameters and methods from knife-ec2 into knife-aws-base (http://github.com/brettcave/knife-aws-base) so that they are available. 

However, knife-aws-base does not have any version constraints, which frees up knife-elb to use independent dependency versions (e.g. fog-1.23 for elb_modify functionality)