Themes!

_Almost_ everything works, just the images (the envelope, checkbox and close images) can't be themed because they're not `svg`. I added the `modal_hue` option to `_bftn_options` object (and added a description in the `README`), and had the hue value put into the iframe src as a hash. The iframe then reads the hash and styles everything that needs styling from an array of the elements, the CSS property that needs styling, and the saturation and lightness values.

I also customized the Gruntfile.js to my liking, feel free to take it or leave it — that was more for myself.
Oh, and the reason it shows some of the images as changed is that the Grunt tasks I have configured losslessly compress the images, so the images are the same, they just load a bit faster.