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.
The only other thing I'd add is that we could probably test the response types of the new operations, i.e.:

```php
public function test_Delete_CertMapping_Returns_Response()
{
   $this->isResponse($this->loadBalancer->certificateMapping(1)->delete();
}
```

Saying that, though, the above is only useful for preserving a high test coverage percentage (which I have mixed feelings about anyway) - IMHO it's not testing anything useful. @ycombinator - what do you think?