chinese: Use method isIntercalaryMonth

* This commit "fixes" a bug in `ChineseCalendar.prototype.add` that
  caused first lunar months to be considered as intercalary. Since this
  is only an issue when adding years, this bug cannot be triggered in
  the range of years currently covered (1888 - 2111).

jquery-src/jquery.calendars.chinese.js

@@ -74,8 +74,7 @@
                         month = "0" + month;
                     }
 
-                    var intercalaryMonth = this.intercalaryMonth(year);
-                    if (intercalaryMonth && monthIndex === intercalaryMonth) {
+                    if (this.isIntercalaryMonth(year, monthIndex)) {
                         month += 'i';
                     }
 
@@ -95,8 +94,7 @@
                     var monthName = ['一月','二月','三月','四月','五月','六月',
                         '七月','八月','九月','十月','十一月','十二月'][month - 1];
 
-                    var intercalaryMonth = this.intercalaryMonth(year);
-                    if (intercalaryMonth && monthIndex === intercalaryMonth) {
+                    if (this.isIntercalaryMonth(year, monthIndex)) {
                         monthName = '闰' + monthName;
                     }
 
@@ -116,8 +114,7 @@
                     var monthName = ['一','二','三','四','五','六',
                         '七','八','九','十','十一','十二'][month - 1];
 
-                    var intercalaryMonth = this.intercalaryMonth(year);
-                    if (intercalaryMonth && monthIndex === intercalaryMonth) {
+                    if (this.isIntercalaryMonth(year, monthIndex)) {
                         monthName = '闰' + monthName;
                     }
 
@@ -372,7 +369,7 @@
             monthIndex = date.month();
             day = date.day();
 
-            var isIntercalary = (monthIndex === this.intercalaryMonth(year));
+            var isIntercalary = this.isIntercalaryMonth(year, monthIndex);
             var month = this.toChineseMonth(year, monthIndex);
 
             var solar = toSolar(year, month, day, isIntercalary);
@@ -422,7 +419,7 @@
         add: function(date, offset, period) {
             var year = date.year();
             var monthIndex = date.month();
-            var isIntercalary = (monthIndex === this.intercalaryMonth(year));
+            var isIntercalary = this.isIntercalaryMonth(year, monthIndex);
             var month = this.toChineseMonth(year, monthIndex);
 
             var cdate = Object.getPrototypeOf(ChineseCalendar.prototype)
@@ -436,7 +433,7 @@
                 // Using the fact the month index of an intercalary month
                 // equals its month number:
                 var resultCanBeIntercalaryMonth =
-                    (month === this.intercalaryMonth(resultYear));
+                    this.isIntercalaryMonth(resultYear, month);
 
                 var correctedMonthIndex =
                     (isIntercalary && resultCanBeIntercalaryMonth) ?