MDL-18375 calendar: only display the YUI2 calendar when the gregorian system is used
MDL-18375 calendar: removed code from the base class and only introduced functionality responsible for generating the date selector 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: removed initial patch changes to core functions
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: declare the userdate function responsible for displaying dates in the calendar type