Enable persistence properties which have only a getter or a setter but not both

[swagner]

For transforming persisted data it would be nice to write persistence properties which have a getter or a setter but not both. Maybe we can simply add an additional flag to the Storable attribute to disable the check that each persistence property has to provide a getter and a setter.

[epitzer]

Implement one-way serialization that allows either only loading or only saving of storable members by setting a new property called AllowOneWay on the [Storable] attribute (r5324)

[swagner]

Great. Thanks.