Replace Schema metaclass with descriptors

[dnephin]

Currently staticconf.schema is implemented using a metaclass. Although this works, it requires a schema to be instantiated, and it's not really obvious what is going.

Descriptors are probably a better fit here. They would remove the need for any base class or metaclass (they could be attached to any class with a namespace attribute), and would allow them to work as accessors on the class instance as well as an instance of the object.