Added
* Added unit keyword arg to Timerit. This forces a Timerit objects to report
time per loop in a particular unit. The default of None chooses a unit.
* The timer returned by Timerit.__iter__ now contains an attribute
called `parent` that references the original timer object.
* Renamed global private attribute `default_timer` to `default_time`
* Timer now uses a `self._time` instead of globally calling `default_time`.
This is mainly to make testing easier.
* Timerit now uses `self._timer_cls` instead of `Timer` for internal timers,
again mainly for testing reasons.
* Add internals to force ascii chars for tests