Implement InternetGateway filters support

All available filters are implemented, and the implementation is based on Instance filtering. More information about IGW filters: http://docs.aws.amazon.com/AWSEC2/latest/APIReference/ApiReference-query-DescribeInternetGateways.html#query-DescribeInternetGateways-filters

Two possible attachment states are *"attaching"* and *"detaching"*, but in moto these states do not exist, and the filter param only accepts *"available"*.