Add support for httpsRedirect flag on Rackspace LB

This PR adds support to the "hidden" `httpsRedirect` flag on Rackspace Cloud Load Balancers. The flag is documented here: http://docs.rackspace.com/loadbalancers/api/v1.0/clb-devguide/content/GET_showLoadBalancer_v1.0__account__loadbalancers__loadBalancerId__load-balancers.html

I'm not sure this patch is ready to merge yet because the `https_redirect` flag is not being correctly set on LoadBalancer instances that come from `list_load_balancers`. This is because the `httpsRedirect` field is not included in the JSON response in that case; only requests to `get_load_balancer` will contain the field. I didn't see a good example of how to handle this in the code. I could imagine the `https_redirect` accessor being augmented to force a call to `get_load_balancer` if the field is used and it hasn't been populated in the initialize call.