Add the base type, that types which want to implement persistency
could embed, and this way inherit persistent functionality. For example

	type MyObject struct {
		Persistent
		...
	}

	type myObjectState MyObject

	func (o *myObjectState) DropState() { ... }
	func (o *myObjectState) SetState(state *mem.Buf) error { ... }

Here state management methods (DropState and SetState) will be
automatically used by persistency machinery on activation and
deactivation.

For this to work MyObject class has to be registered to ZODB

	func init() {
		t := reflect.TypeOf
		zodb.RegisterClass("mymodule.MyObject", t(MyObject{}), t(myObjectState))
	}

and new instances of MyObject has to be created via zodb.NewPersistent:

	obj := zodb.NewPersistent(reflect.TypeOf(MyObject{}), jar).(*MyObject)

SetState corresponds to __setstate__ in Python. However in Go version it
is explicitly separated from class's public API - as it is the contract
between a class and persistency machinery, not between the class and its
user. Notice that SetState takes raw buffer as its argument. In the
following patch we'll add SetState cousing (PySetState) that will be
taking unpickled objects as the state - exactly how __setstate__
operates in Python. Classes will be able to choose whether to accept
state as raw bytes or as a python object.

The activation/deactivation is implemented via reference counting.

Tests are pending (for PySetState).