Fix for Issue#1417

As per :
http://docs.aws.amazon.com/ElasticLoadBalancing/latest/DeveloperGuide/USVPC_DetachSubnet.html

The API call should be "DetachLoadBalancerFromSubnets" and not "DettachLoadBalancerFromSubnets"
Adding unit test for detach_internet_gateway
pep8 standards
Changing to unittest2
Changing call  to super for setUp
using from clause to import TestCase