Shouldn't hook activation function directly

[JDGrimes]

In our bootstrap we hook up the activation function directly. However, this is silly, because it should automatically be hooked up by register_activation_hook(). It is also unhelpful because it means that the tests won't catch an issue with the call to register_activation_hook() within the plugin (WordPoints/wordpoints#349).

I think the reason that I did it this way is because of issues with properly applying plugin_basename() to the plugin path. Let's see if we can resolve that somehow.