Add SCSS Support

Added SCSS support, cloned from `leaflet.awesome-markers.css` with a bonus (and intended) side effect that you can now specify where the plugin's icon images live via the `$awesome-marker-icon-path` variable (which defaults to the CSS version of `images/`).

Also added a `bower.json` to make Bower happy when installing the plugin.

Also also updated the `README.md` with steps to use the SCSS version as well as the CSS version.