Add glyphs/calendars example

This adds a calendar for 2014, which is a grid plot of 12 categorical plots. Includes US holidays (depends on `icalendar` which I currently install with pip; waiting for conda package). If someone wonders why this is called `calendars.py` not `calendar.py`, then the short answer is not because I plan to add support for choosing the year (perhaps I will, but that's irrelevant). Hint: imports in Python are "awesome".