New interface for advanced phpunit testing and phpunit testing on the firebaseLib

Tamas,

Thanks for your work on the firebase-php library. You really saved me on a project I am working on.

In my project, I wanted to use phpunit to test without any latency. As such, I wrote an interface to be consumed by both the firebaseLib and a new firebaseStub that I created for phpunit testing.

In the course, of adding the firebaseStub, I noticed that your unit tests did not actually verify the results from firebase. So, I converted your tests into phpunit tests. To make all of this happen, I need to add a few directories: lib and tests.

I also added an additional validation for the baseURI during the class initialization.

In addition, I updated the README file to include all of the changes with explanations of how to use the Stub for testing.

Let me know, how I can help merge this code.