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.

[nosetests]
exclude=pymox
with-coverage=1
cover-package=soc
cover-erase=1
cover-html=1
cover-html-dir=coverageResults
#debug=nose.plugins.cover