MDL-18375 calendar: removed code from the base class and only introduced functionality for the date form elements

In the original patch we are overwriting core functions responsible for saving time in the
database in the calendar base class. This is very dangerous, we do not want to touch these
functions as it could cause a complete fubar of the database. There are places we are forcing
the use of the gregorian calendar as we are passing dates generated by the PHP date function,
where as sometimes we pass dates from usergetdate (which was being overwritten to return the
date specific to the calendar). We can not expect third party modules to change the calendar
system depending on the format they pass to these functions. In future commits I will be
expanding the base class when needed and removing any changes to the core functions when
related to the commit.
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: moved the location of the calendar type select box when editing a course
MDL-18375 calendar: added the userdate and usergetdate functions to the calendar type
MDL-18375 calendar: edited the calendartype column in the course and user table in the install.xml file and created an upgrade path
MDL-18375 calendar: refactored the datetime user field so that it now uses the calendar type system
MDL-18375 calendar: implemented the usage of the recently introduced automatic class loading
MDL-18375 calendar: created a test calendar type to use for PHPUnit tests and added initial PHPUnit tests
MDL-18375 calendar: changed the order of the parameters passed to conform to other core functions
MDL-18375 calendar: removed uses of PHP date functions not present on all PHP installs

Changed the logic of the test calendar so that it does not use the
Julian date calendar as the PHP date conversion functions (eg. jdtogregorian)
are not always present on all PHP installs. Also split the unit test
into multiple tests and rearranged the order parameters were passed to
certain functions as they were changed in the previous commit.
MDL-18375 calendar: added a PHPUnit test to ensure the datetime user profile field settings are saved correctly
MDL-18375 calendar: made userdate and usergetdate abstract to ensure developers implement this functionality in their calendar type
MDL-18375 calendar: added '\' before each use of the core_calendar namespace
MDL-18375 calendar: removed unnecessary functions for converting years for the datetime user profile field and use existing functions instead
MDL-18375 calendar: renamed the userdate/usergetdate function in the calendar type to be more descriptive

Also tidied up PHPDocs for these functions.
MDL-18375 calendar: PHPDoc tidy up as well as some other minor changes
MDL-18375 calendar: only display the choice of calendar types if there is more than one
MDL-18375 calendar: added the Gregorian calendar type to the standard list of plugins
MDL-18375 calendar: added condition to ensure calendar type exists before loading
MDL-18375 calendar: changed the name of the function factory() to be more descriptive