Add grunt-image-embed to embed data uri's in CSS

\#28 should be reviews and merged first, as this is based off that.

Added the first bit of a build command that is later intended to build all the required icons and formats from a single SVG heart-coin image.

Using SVG's for the data uri was suggested by @rummik, but there already exists a simple little grunt tool for embedding base64 encoded images. Otherwise we'd need to write our own, and I don't see the benefit, but feel free to correct me if I'm missing something :)