Message Signature Validation

[Pivotal Issue 78632442][1]

[1]: https://www.pivotaltracker.com/n/projects/1163646/stories/78632442
A bit more exanding on the commit comment: The `tests/` directory uses the same namespace as the `src/` directory, which causes issues when trying to load mocks that are in the tests directory. `SAML2_SomeMockedThingyMaBob` would only be found in the `src/SAML2/` dir. This allows it to retry in the `tests/` directory, so that mocks can be kept near the tests rather than in the source as is done with the MockContainer. Do note that it is only in the test autoloader and *not* in the composer autoloader, so it will only work when actually using this autoload file, whcih is only done through the phpunit config (it is loaded as bootstrap).