Allow the specification of custom endpoints.

Hi,

This one allows the specification of a custom endpoint to talk to. See the changes in README for an example. 

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.  Support for awssum-amazon-s3 will follow soon.

Please review them and consider rolling them into the main distribution.

Thanks,
Giannis