Added ability to setTestMode to true/false

I was getting API auth failures with this class, and finally worked out it was because it was hitting up the live API. I had a key for the test API. I've added a function called "setTestMode", so the user can set the mode to true/false:

$gateway->setTestMode(true);

If it's true, it'll use the dev API. A false will use the live API.

Previously, the other way of doing it was going into the class and changing it manually. This change gives options.