Skip thumbnail creation for SVG files - use the SVG itself instead

This fixes #641 and #579. I had to use an actual `Blob` in the tests or else `FileReader` would fail and there would be no way to check the result of `createThumbnail()`. Unfortunately support for `Blob`s is not consistent across browsers so I also had to use an ugly shim to create the `Blob`. I'm not familiar with Chai/Mocha, so I'm not sure if there's a better solution than just including the shim function inside the `it` block or not.

I know that the `CONTRIBUTING.md` indicates that discussion should happen prior to pull requests, but this issue was breaking functionality for one of our clients so I was directed to get a forked solution working ASAP. If I'm going in the wrong direction with this or if there's anything else I should be doing to make this more helpful I'm all ears.