Add SVG support to openbadges

* Added lib/svg.js to parse SVG images, read and write badge data + new tests
* Added lib/image.js to parse image buffers and detect mime types + new tests
* Updated content-type assertion to detect multiple expected MIME types
* Updated prepare() and getDataFromImage() in lib/baker.js to detect image MIME type and run new code for SVG images
* Added image MIME type detection to lib/award.js to help determine appropriate file names
* Added SVG tests based on existing PNG tests
* Added lodash to package deps (underscore `contains()` seems to not work properly on strings)