Option to exclude API base classes

[maxrice]

As wonderful as the API base classes are, not every extension makes use of them. In an effort to slim down the size of the framework when included, we should add an initializer option for extensions to declare whether the API files should be loaded or not, similar to the payment gateway option.

[justinstern]

we're going to run into the same chicken-and-egg problem that always comes up with trying to add functionality to the bootstrap class. This is something we're probably going to have to figure out a general solution for, one way or the other. Might end up being a matter of reworking the bootstrap class to support what we now know is needed, with some sort of one-time major backwards incompatible release, that forces people to either update all frameworked plugins and get the updated bootstrap, or deactivate older plugins

[bekarice]

If we do consider this for a backwards-incompat fw release, perhaps considering #175 at the same time would be warranted