Add support for prefetching fields
This makes it possible to specify the fields that should be
prefetched as part of a getForFields query. Doing a prefetch on a
ReferenceProperty when it is known that the property will be
referenced in (almost all) entities is far more efficient than
doing the invididual fetches sequentially.
import os, sys
sys.path.insert(0, os.path.dirname(os.path.dirname(__file__)))
import pkg_resources
pkg_resources.require('WebOb')