Added support for downloading PNG file along with SVG file. 

I have added a new file called svg-crowbar-png.js
In the above file I have added a function called downloadPNG() which uses the SVG source to create a PNG file ready for download using the canvas. This change also adds a new button called "Download PNG" and changes the original download button to "Download SVG" and gives user option to choose either downloads.

The changes also include a new link for bookmarklet which adds the link to the svg-crowbar-png.js file served from CDN.rawgit.com

All the changes can be seen at the demo on: http://shubhanshu.com/svg-crowbar/