Adds a new Model Spree::PropertyType. Spree::Property belongs to a property type. The property types has name, presentation an icon. Each property type can be shown in diferente places in the store:
left side: show properties in the product description, under de images, organized by types
right side: Under de product description
index: show properties in the product index
each property can be shown in different ways:
name - value: this is the default way, the current way
name only: this only shows the name of the properties
icon - value: show the icon and the value
icon only: show only the icon