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.
@justmarkup I agree, add the CSS that applies to the child `svg` into the library, based on the `options.responsive` flag.