Implement InternetGateway filters support

More information about IGW filters: http://docs.aws.amazon.com/AWSEC2/latest/APIReference/ApiReference-query-DescribeInternetGateways.html#query-DescribeInternetGateways-filters
Move filter logic from responses.InternetGateways to models.InternetGatewayBackend
Move filter logic from responses.InstanceResponse to models.InstanceBackend
Merge pull request #254 from spulec/internet-gateway-filters

Implement InternetGateway filters support