Take into account custom ports

Hi,

this patch passes custom port info into the signing process.
 
I wrote it in order to use awssum libraries against third party services supporting the AWS EC2 API, like OpenStack, Eucalyptus etc. I am using it successfully with OpenStack as well as Amazon EC2.

It comes together with pull requests for awssum and awssum-amazon-ec2. Support for awssum-amazon-s3 will follow soon.

Please review them and consider rolling them into the main distribution.

Thanks,
Giannis