Refactor PersonAttributeType to use a "datatype" and "handler_config" instead of a "format"

The way we've implemented PersonAttributeType.format is quite hacky. That property stores the fully-qualified name of the Java class that gets stored (in String form) in PersonAttribute.value. It's up to the class to convert itself to/from String by implementing Attributable.

Instead we should implement the idea of AttributeHandler (discussed in TRUNK-2131), which should be a shared resource with VisitAttributeType and other AttributeTypes.

Thus a PersonAttributeType would specify an "attribute datatype" and a "handler config", which together describe how to instantiate a handler. The handler would control how the PersonAttribute.value column is converted between a String and a java object.