added POST request utility method

I was using quiche (really nice to work with), and bumped into the 2040 character limit on GET requests.
POST requests can handle way more data points (up to 16k characters I believe), so I added a little method that returns a basic `require(['http' || 'https']).request` POST request pointing at your chart.

I added it to url.js because it uses the same resources so I changed the name to api.js (now containing both GET url and POST request methods)

So now I do
```javascript
var tofile = fs.createWriteStream('chart.png')
chart.getPostReq(true, function(res){
  res.pipe(tofile)
}).end()
```
and I have my (big) chart!

Thanks for quiche and have a good day,

Marcus