MDL-18375 calendar: added multiple calendar support

Conflicts:

	course/edit_form.php
	lib/db/install.xml
	lib/form/dateselector.php
	lib/form/datetimeselector.php
	lib/setup.php
MDL-18375 calendar: removed calendar systems that should be downloaded as plugins
MDL-18375 calendar: moved the calendar systems location to a sub-folder in the calendar directory
MDL-18375 calendar: removed language for the gregorian calendar system that should be downloaded separately from core

Also removed unnecessary version.php file.
MDL-18375 calendar: removed custom files responsible for checking for updates as this should be done through core