Basic calendar plugin. Provides yearly, monthly, weekly, and daily views
for any date. Calendar event support is in progress. Adding/editing/deleting
events is functional but there are still issues to address. Event support
requires that Hastymail be configured to use a database. This can be configured
in the hastymail2.conf file. It supports both postgres and mysql.
Table definition files for each can be found in the plugin directory:

plugins/calendar/calendar.mysql.sql
plugins/calendar/calendar.psql.sql

TODO as of 1/13:
    - add duration, maybe other detail to day view
    - fix week "month wrap around" bug
    - fix width display bug (interface does not adjust to
      calendar width when the calendar is wider)
