Throw error on API override in action/store creation

As [suggested](https://github.com/spoike/refluxjs/pull/67#issuecomment-55997728) by @spoike, this PR makes action/store creation fail fast when you try to override API methods, instead of just silently ignoring the code.
:+1: for guard statement check

Is there some reason for throwing a string rather than `throw new Error("Cannot override API method in action creation[...]");`? Do you still have a stack trace if you're throwing a string?