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.
In addition to @gabrielfagundez CSS, we also need to set width and height to 100% for the svg, otherwise it breaks if you resize from small to big.

```
#container { 
    position: relative; 
    padding-bottom: 60%;
}

#container > svg {
    position: absolute;
    width: 100%;
    height: 100%;
}
```

I suggest including this CSS directly in datamaps, but only if a user explicitly sets ```options.responsive```, which should be false for now. The reason behind is that if someone uses eg. ```<div id="container" style="position: relative; width: 500px; height: 300px;"></div>``` at the moment and updates datamaps it will break his map with the new CSS. 

@markmarkoh what do you think?