Prevent setState calls in submit if unmounted after validation

[jaredpalmer]

Fixes #854 by checking for this.didMount after async validation is done

[jaredpalmer]

@chasecaleb does this work?

[chasecaleb]

I think it should actually be this:

if (isValid) {
    this.executeSubmit();
} else if (this.didMount) {
    this.setState({ isSubmitting: false });
}

That way the submit will still happen when unmounted, but without the broken call to setState().

[jaredpalmer]

Will cut a release when this passes.