lib: introduce string conversion functions for ASCII without further encoding concerns Avoid the trial-and-error and vagueness of the "safe" functions. This should replace some use of safe_unicode and safe_str. It will mostly be a noop in py2 but will be crucial in py3.

[tox]
minversion = 1.8
envlist = py27-pytest

[testenv]
setenv =
    PYTHONHASHSEED = 0
deps =
    -r{toxinidir}/dev_requirements.txt
    python-ldap
    python-pam
commands =
    pytest: py.test {posargs}