Allow names in crop URLs

Implements feature request #118. Like @alpha-ninja proposed, anything can be put in the name. Search results will use the name of a crop lowercased and with non-alphanumeric characters replaced by “-”, so for example “yellow Streich” becomes http://openfarm.cc/crops/yellow-streich/54106525446576621e010000 This makes it easier to post these links to web forums or send them by email, because whitespaces in the URL would break the link and it wouldn't be clickable.