Add get + set for $apiBase to increase testability #27

We're currently integrating the ConvertAPI into our project. When writing a unit test, we tried to mock the ConvertApi class. This works fine to a certain extend. In a piece of code we have the following:

ConvertApi::$apiBase = 'eu-v2.convertapi.com';

We use PhpUnit and Mockery to write tests, and currently mocking static properties is not supported. After some experimenting, we found the best solution would be to add a setter (like setApiSecret) so that we can finish our unit test. I think other people can also benefit from this, as it's not possible to write a unit test if you set a custom apiBase.

Thoughts about this? I can open a PR if there's interest in this.
PR here: #28