Add option to specify endpoint's signature version

[kyleknap]

Add option to pass signature_version to EndpointCreator's create_endpoint method.

When get_endpoint is called from Service objects, it will pass in its signature version to create_endpoint. That is required for the CLI's --no-sign-request flag to work because the handler changes the service object's signature version.

I will be sending a PR out real soon for the CLI, that will add a test to catch that --no-sign-request is adhered to.

cc @jamesls @danielgtaylor

[coveralls]

Coverage increased (+0.05%) when pulling 3d9ae51 on kyleknap:no-sign into 38faa0e on boto:develop.

[kyleknap]

@jamesls Test now pass. I added a flag to check if the service object is using a custom signature version.

[jamesls]