[aws] [elb] Support cross-zone load balancing

Allow for EC2 ELB Load Balancers to use the new cross-zone load balancing
feature which better distributes load across all Availability Zones.

http://aws.amazon.com/about-aws/whats-new/2013/11/06/elastic-load-balancing-adds-cross-zone-load-balancing/

Disabled by default unless you add "cross_zone_loadbalancing true" to your ELB definition.

To support this feature, Fog needed to be bumped to 1.19.0 (and excon to 0.31.0).  Specs pass, and Things Work For Me(tm), but caution is advised.   One side-effect is that the error message for duplicate security groups changed, so I have adjusted that throughout the code.