Significantly flesh out the `/calendar` URI.

Populate the calendar with the correct number of day cells for the month being
shown. Place the day number in the upper corner of each day cell and style it
appropriately. Give each day cell a class such as "mon" or "tue", ensuring that
the CSS rules which push the cells into the correct columns can take effect.
Make those CSS rules more robust: multiple cells can be given a class such as
"mon" or "tue" without adverse effect.

Add a CSS rule which applies a "clearfix" to any element with the class
`floatcontainer`. (see http://csscreator.com/attributes/containedfloat.php)
Apply class `floatcontainer` to the second `<ol class='calendar'>`, and remove
the `<div style='clear:both;' />` from the same `<ol>`. This allows the
`/calendar` URI to validate as proper HTML5.

Remove template tag `day_names`, as it promotes a too-large
infrastructure-to-work ratio. Day names are now generated directly within
`elts.views.calendar` and passed to the `elts/calendar.html` template.

Rename variables in `elts.templatetags.calendar_tools` to make them more
distinct from module names. Create template tag `day_name_abbrev` to generate
class names for each cell in the calendar. Also create template tag `month_days`
to assist in generating numbers foe each cell in the calendar. (perhaps it
should be renamed to `day_numbers`?)