adding connection draining and cross zone load balancing options to ec2_elb_lb

Please let me know if there are any issues with my pull request. I have added a feature to configure ELB attributes. I have done cross zone load balancing and connection draining. I did not implement accessLog as I have no need for that ATM. More information can be found http://boto.readthedocs.org/en/latest/ref/elb.html#module-boto.ec2.elb under modify_lb_attribute.