Allow custom port and path to be set as option

Hi,

this little patch allows for custom port and custom path to be set as options to the service. 
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-amazon 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