Added instructions to the Readme about adding a require to your store/all.css manifest file.

Renamed store/screen.css.scss to store/spree_blue_theme.css.scss to be consistent with the
spree_blue_theme gem.
Fix formatting of code blocks in Readme
Match the changes to partials made in spree core
Fix to look good with Spree 1.3
Added instructions to the Readme about adding a require to your store/all.css manifest file.

Renamed store/screen.css.scss to store/spree_blue_theme.css.scss to be consistent with the
spree_blue_theme gem.