Extend beforeShowMonth event functionality

The event `beforeShowMonth` could only disable the cells.
I extended it with the functionality of `beforeShowDay` so you can disable, add classes and set a tooltip.
Backwards compatibility is given.

Author: tpeter1985

js/bootstrap-datepicker.js

@@ -1103,12 +1103,20 @@
 			if (this.o.beforeShowMonth !== $.noop){
 				var that = this;
 				$.each(months, function(i, month){
-					if (!$(month).hasClass('disabled')) {
-						var moDate = new Date(year, i, 1);
-						var before = that.o.beforeShowMonth(moDate);
-						if (before === false)
-							$(month).addClass('disabled');
-					}
+          var moDate = new Date(year, i, 1);
+          var before = that.o.beforeShowMonth(moDate);
+					if (before === undefined)
+						before = {};
+					else if (typeof(before) === 'boolean')
+						before = {enabled: before};
+					else if (typeof(before) === 'string')
+						before = {classes: before};
+					if (before.enabled === false && !$(month).hasClass('disabled'))
+					    $(month).addClass('disabled');
+					if (before.classes)
+					    $(month).addClass(before.classes);
+					if (before.tooltip)
+					    $(month).prop('title', before.tooltip);
 				});
 			}