added 'main' array pointing to css & font files for better integration with build systems

I was attempting to use roboto-fontface-bower with gulp-bower-files, but it failed with the error 'The bower package jfm-roboto-fontface has no main file(s)'. After adding the main array to bower.json, it is able to find the required files and proceed without having to provide gulp-bower-files with the file locations in an overrides property. I assume that this update will also allow robot-fontface-bower to better work with other build systems like Brunch.