MDL-18375 calendar: renamed and set directory name to singular to match other Moodle plugins

Also did some minor changes such as removing/moving language strings.
MDL-18375 calendar: added the userdate function to the calendar type
MDL-18375 calendar: added upgrade path to install the calendartype column in the course and user table
MDL-18375 calendar: added the usergetdate function to the calendar type and an additional abstract function