prettify logging

Alternatively, modify the logger in place so you can do ``log.time`` (this might be a horrible idea)

``` python
def add_timing_logger(logger, precision=3, level=logging.DEBUG):
    """ ...

    Usage::

        >>> logger = logging.getLogger('foobar')
        >>> add_timing_logger(logger, level=logging.INFO, precision=2)
        >>>
        >>> with log.time("hello %s", "world"):
        ...     time.sleep(1)
        INFO:foobar:hello world in 1.00s
    """
    log_method = partial(logger.log, level)
    log.time = partial(_log_time, log_method, precision)
```
if you're refactoring anyway, why not just pass the level down into `_log_time` instead of faffing with `partial`?