building now uses $SPHINX_BUILD_TAGS env var.

realized can do this now that we're requiring python3 >= 3.3.

had to keep u() helper around for u(r"") instances, since py3 doesn't have ur"".
but switched it to use ascii decoding for py2, to make sure things are clean.

will be removed completely in 1.8.

Rationale:
Aside from the arguments in issue 83, performed a timing analysis,
and decided harden_verify framework wasn't going to be easily workable
to prevent a timing attack anyways (see attached admin/plot_verify_timing.py script).

Changes:
* dummy_verify() has been kept around, but now uses .verify() against a dummy hash,
which is guaranteed to have correct timing (though wastes cpu cycles).

* Removed most harden_verify code, treating it as NOOP just like min_verify_time.
  Similarly, removed most documentation references to.