Use an Em.computed property for LineItemsShowController#canUpdate

As per https://github.com/radar/spree/pull/16#discussion_r12266793
Use better CoffeeScript syntax in LineItemsShowController

As per https://github.com/radar/spree/pull/16#discussion_r12266798
Move payment firing into the Payment model

As per https://github.com/radar/spree/pull/16#discussion_r12266808