Allow to set the text color for calendars, too

Events belonging to a certain calendar are displayed in a color that is defined on the calender page.

Currently only the background color can be set, and not the font color; the latter always defaults to white. This is hard to read against light background colors like yellow.

As the FullCalendar.js also supports setting the font color a new field "font color" should be added to the calendar class; defaults to white for backwards compatibility.