Added READMEs to each language client library test

This pull request consists of the follow changes:

* Added the existing AWS PHP SDK test suite to the `Makefile`
* Added more consistent READMEs to each language's client library test suit

Despite the fact that most of these tests can be driven by the `Makefile`, I READMEs to each so that anyone who tries to learn how to use Riak CS from the tests has an easier time.