Add Support for Certificate Mappings

## Summary

The PHP SDK for the Rackspace API lacked support for the new certificate mappings feature recently released. 

## Approach

A couple of folks (@JPry and @ttaylor-wpe) and myself worked together to add support to the SDK. 

## Risk

Low

It has been tested and introduces new functionality. We have seen no negative side effects with these code modifications.

## Tests

New unit tests have been added to verify that the URL being hit for the API calls matches what is expected.
Could we rename this to `CertificateMapping` since the class represents a singular entity?