Convert commerce_kickstart_theme to a subtheme of omega_kickstart

Currently, omega_kickstart and commerce_kickstart_theme are so close visually that we need to duplicate most of the visual changes that we make.

Omega Kickstart has the goal:

This theme is meant to be subthemed and become a starting point for your own theme development that is a little more friendly out of the box than Omega by itself.

It makes sense for commerce_kickstart_theme to be one of those subthemes. This provides an example to the themers (showing them how it should be done) while allowing us to reduce the duplication of CSS and further work.
While doing this, we should make sure that all CSS that remains in commerce_kickstart_theme has a SASS equivalent (since that's how omega_kickstart is developed, SASS first, then an automatic conversion into CSS).