Apply a png image onto another png image with given coordinates
var Watermark = require('./watermark.js');
var watermark = new Watermark('./image_src/image.png', function () {
var x = 15;
var y = 15;
var opacity = 50;
this.push('./image_src/watermark.png', x, y, opacity, function(){
this.save('./image_src/result.png');
console.log('done');
})
var cropped = this.crop(0, 0, 100, 100);
cropped.save('cropped.png')
});