Added snipcart stylesheet, added snipcart-button directive.

I added the snipcart stylesheet that was missing. I created a directive that generates Snipcart buy buttons and made a small CSS trick on the styles.css stylesheet to override default Snipcart z-index.

There is still something that does not work well, the routing between the Angular app and Snipcart are conflicting, I will take a look on Snipcart side if there is anything we could do.