Dynamically generate calendar title.

At the top of the calendar at URI `/calendar`, dynamically display a title. For
example: "January 2013". Write template tag `calendar_title` to facilitate this
change and add a corresponding doctest.
Dynamically generate calendar week day names.

At the top of the calendar at URI `/calendar`, the name of each week day is
displayed. Dynamically generate those week day names. Write template tag
`day_names` to facilitate this change and add a corresponding doctest.

Rename template tag `calendar_title` to `month_and_year`.