Implement org.freedesktop.DBus.Properties

[guelfey]

1. Create a structure that implements the following interface:

   type Properties interface {
       Get(iface, property string) (Variant, *Error)
       Set(iface, property string, newv Variant) *Error
       GetAll(iface string) (map[string]Variant, *Error)
   }

   Usage:

   var prop Properties
   // ... 
   conn.Export(prop, "/foo/bar", "org.freedesktop.DBus.Properties")

2. Find a good way to make the properties available from the Go program (define methods that take/return interface{} instead of Variant? let the user give all properties upon creation or make it possible to modify them at runtime?)