Check type of payload creator

[globalchubby]

This takes care of the first example in the createAction spec outlined here.

This contains breaking changes, so it might be worth including in #122.

[timche]

With the increase of checks and errors, I'd suggest to use invariant. It is already optimised for development and production with checking process.env.NODE_ENV.

We can start to using it in this PR and open a new PR for all other errors.

[globalchubby]

Using invariant per your suggestion now.

Can you explain the behavior when process.env.NODE_ENV === 'production? I'm not familiar with this library and couldn't quite figure it out via the README.

From my point of view the main benefit is reducing the boilerplate of throwing errors on certain conditions.

[globalchubby]

For the invariant usage, I decided not to go with this approach, since the original error message is actually shorter than the "minified exception" message.

[globalchubby]

@timche is it possible to change the base branch to v1.0.0?

[timche]

Yes, I changed it.

[BerkeleyTrue]

~~~I've always used `null` when I want to omit an argument instead of undefined. I'm still hesitant to use `undefined`.~~~

After some further research, I've decided to move to using `void 0` in my code.

[timche]

LGTM!

[globalchubby]

@timche with this merge, v.1.0.0 is good to go.