Properly add yield-from, using python's real "yield from".

The yield-from that existed previously wasn't actually implementing the
full complexity of "yield from":

  http://legacy.python.org/dev/peps/pep-0380/#formal-semantics

... this includes passing along errors, and many other things.

Also removes the yield-from backport macro, since it does not seem
possible at present to conditionally build macros.

Thus, there is no longer yield-from on pre-python-3.3 systems.

Includes updated docs and tests to reflect all this.