add map.resize() to make map responsive

see #12 

This PR adds the function map.resize() which can be attached to an eventlistener, eg:

```
window.addEventListener('resize', function(event){
    map.resize();
});
```

I didn't add a resize eventlistener directly into datamap cause this way people can decide when they want to call map.resize() as some use smartresize or the like.

I also didn't add the CSS to make the svg responsive directly in datamap, you can see an example CSS in src/responsive.html

@markmarkoh I am not sure if I coded everything according to your preferences, so if you have any questions, improvements... let me know.
Do we need a `options.responsive` option at this stage?

I could see it being useful if we were the ones setting up the `window.onresize` handler, but since the implementer has to decide when to call `resize`, I think we can remove this `if` check and that option.

Basically - whenever somebody asks `Datamaps` to `resize`, we should do it. It's rare that a user will set `responsive` to `false` but continue to call `resize`.