ENH: Configure proper logging.

This addresses #183.

This PR creates a logger at the module level and uses it where print calls (to be precise, `print_update` calls) were previously used. A function, `handle_logging()`, called at import time, does logging configuration to create the same user experience we have now. But users can turn off or configure logging by accessing `trackpy.logger`. They can restore "factory default" logging -- undoing the actions of `handle_logging()` -- by calling `ignore_logging()`.

Discuss!