Create/Delete/Describe for network acl

Implemented following APIs for network acls. 

[Create Network ACL](http://docs.aws.amazon.com/AWSEC2/latest/APIReference/ApiReference-query-CreateNetworkAcl.html)
[Delete Network ACL](http://docs.aws.amazon.com/AWSEC2/latest/APIReference/ApiReference-query-DeleteNetworkAcl.html)
[Describe Network ACL](http://docs.aws.amazon.com/AWSEC2/latest/APIReference/ApiReference-query-DescribeNetworkAcls.html)


This is required for adding network acl support in Terraform, which I am currently working on.
Could we make this (and other comments) more verbose and explain what the function does. These comments are used to generate the GoDocs, which many users view for the API.