add calendar property to allow customization

SORelativeDateTransformer/SORelativeDateTransformer.h

@@ -42,6 +42,13 @@
  */
 + (NSValueTransformer *) registeredTransformer;
 
+/**
+ @brief Calendar used to compute date relative difference.
+
+ +[NSCalendar autoupdatingCurrentCalendar] will be used as default calendar if not set.
+ */
+@property (nonatomic, strong) NSCalendar *calendar;
+
 /**
  @brief Transform an NSDate into a phrase expressing the relative difference between that date and now.
  @param value An NSDate to be compared to the current date.

SORelativeDateTransformer/SORelativeDateTransformer.m

@@ -10,6 +10,8 @@
 
 @implementation SORelativeDateTransformer
 
+@synthesize calendar = __calendar;
+
 + (NSValueTransformer *) registeredTransformer
 {
     return [NSValueTransformer valueTransformerForName:NSStringFromClass(self)];