implemented CreateNetworkAcl
implemented CreateNetworkAclEntry
Implementec Describe network ACL
renamed DescribeNetworkAcls to NetworkAcls
implemented DeleteNetworkAcl
Delete network acl entry
formatted code using gofmt
fixed comments for functions
NetworkAclEntry accepts protocol as int

AWS API is misleading. Docs mention that NetworkAclEntry.Protocol is
int, but examples uses string.Fixed examples to use string.
added ReplaceNetworkAssociation contract
Merge pull request #153 from snehaso/network_acl

Create/Delete/Describe for network acl