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 might even say this is a debug level statement, according to the docs warn -> "An indication that something unexpected happened, or indicative of some problem in the near future (e.g. ‘disk space low’). The software is still working as expected.".  This is both expected _and_ indicates improvement in the future!