Be able to login on signup

So to close #83 I wrote this patch.

Now we can configure whether we want to login on signup or not. By default it does (which is the most common practice this days).

I also updated the node example (with my low node knowledge, so maybe is not the best solution).

the signup always return the response back, even if you don't want to login. That gives flexibility to the end user.

README is also updated :+1: 

Otoh, I think that the login/signup methods are a little verbose with the promises, but that is for another PR and I also need to code some tests.