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!
I vote for INFO-level. Via the docs, that is "an indication that everything is working correctly." I worry that users will suspect hung execution or impossibly slow execution and abandon ship. This can take quite a long time on some systems.