Added support for downloading PNG file along with SVG file.

[napsternxg]

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/

[hugolpz]

@napsternxg : it may be relevant to rather embed the raster. See #16

[techtonik]

Uncaught ReferenceError: d3 is not defined

[techtonik]

A very useful feature. For example to paste diagrams like this into Github you need to convert it to PNG.

[dfan]

There's a DOM 5 in Safari when the image source is encoded to base64 (line 247 in napster's pull request) because Safari doesn't like escape characters but Chrome is okay with it.
This will work instead:
Original: 'data:image/svg+xml;base64,'+ btoa(source.source);
New: 'data:image/svg+xml;base64,'+ btoa(unescape(encodeURIComponent(source.source)));