add ability to clear time

Enables timepicker to be reset with clear or setTime(null)

ex.
$('.timepicker').timepicker('setTime', null);

or

$('.timepicker').timepicker('clear');

fixes #155

js/bootstrap-timepicker.js

@@ -90,6 +90,15 @@
       this.updateFromElementVal();
     },
 
+    clear: function() {
+      this.hour = null;
+      this.minute = null;
+      this.second = null;
+      this.meridian = null;
+
+      this.$element.val('');
+    },
+
     decrementHour: function() {
       if (this.showMeridian) {
         if (this.hour === 1) {
@@ -228,11 +237,11 @@
     },
 
     formatTime: function(hour, minute, second, meridian) {
-      hour = hour < 10 ? '0' + hour : hour;
-      minute = minute < 10 ? '0' + minute : minute;
-      second = second < 10 ? '0' + second : second;
+      hour = hour !== null ? (hour < 10 ? '0' + hour : hour) : '';
+      minute = minute !== null ? (minute < 10 ? '0' + minute : minute) : '';
+      second = second !== null ? (second < 10 ? '0' + second : second) : '';
 
-      return hour + ':' + minute + (this.showSeconds ? ':' + second : '') + (this.showMeridian ? ' ' + meridian : '');
+      return hour + (hour !== '' ? ':' : '') + minute + (this.showSeconds && second !== '' ? ':' + second : '') + (this.showMeridian && meridian !== null ? ' ' + meridian : '');
     },
 
     getCursorPosition: function() {
@@ -666,6 +675,11 @@
       var arr,
         timeArray;
 
+      if (!time) {
+        this.clear();
+        return;
+      }
+
       if (this.showMeridian) {
         arr = time.split(' ');
         timeArray = arr[0].split(':');

spec/js/TimepickerSpec.js

@@ -389,6 +389,18 @@ describe('Timepicker feature', function() {
     expect(hideEvents).toBe(1);
   });
 
+  it('should be able to reset time by using setTime 0/null', function() {
+    tp1.hour = 10;
+    tp1.minute = 30;
+    tp1.meridian = 'PM';
+    tp1.updateElement();
+
+    $input1.timepicker('setTime', null);
+    tp1.update();
+    expect(tp1.getTime()).toBe('');
+  });
+
+
   it('should not have the widget in the DOM if remove method is called', function() {
     expect($('body')).toContain('.bootstrap-timepicker-widget');
     tp1.remove();