patch unifying pickling handling. This path must wait until zodbpickle works on python2.7 and OS X
make cached _len the standard, drop uncaphed version
try to import zodbpickle.fastpickle.

hoping that my changes will be accepted, otherwise it does not harm.
Merge branch 'use_zodbpickle'

* use_zodbpickle:
  try to import zodbpickle.fastpickle.
  patch unifying pickling handling. This path must wait until zodbpickle works on python2.7 and OS X
clean-up the package
move pickle error handling to utils
correct the node._len implementation (hard)
better setup and cleanup, PyPy awareness