Change the way we test relative months and years in RelativeDateTest

1. Tests for relative months and years are now executed against fixed
   test dates.
2. Add additional test for "special cases".
3. Add test for leap years.

test/RelativeDateTest.py

@@ -43,12 +43,40 @@ class RelativeDateTester(TopydoTest.TopydoTest):
         self.assertEquals(result, date.today() + timedelta(weeks=1))
 
     def test_one_month(self):
-        result = relative_date_to_date('1m')
-        self.assertEquals(result, date.today() + timedelta(30))
+        test_date = date(2015, 1, 10)
+        result = relative_date_to_date('1m', test_date)
+        self.assertEquals(result, date(2015, 2, 10))
+
+    def test_one_month_ext(self):
+        test_date1 = date(2015, 1, 29)
+        test_date2 = date(2016, 1, 31)
+        test_date3 = date(2015, 12, 31)
+        test_date4 = date(2015, 7, 31)
+        test_date5 = date(2015, 10, 31)
+
+        result1 = relative_date_to_date('1m', test_date1)
+        result2 = relative_date_to_date('1m', test_date2)
+        result3 = relative_date_to_date('1m', test_date3)
+        result4 = relative_date_to_date('1m', test_date4)
+        result5 = relative_date_to_date('1m', test_date5)
+
+        self.assertEquals(result1, date(2015, 2, 28))
+        self.assertEquals(result2, date(2016, 2, 29))
+        self.assertEquals(result3, date(2016, 1, 31))
+        self.assertEquals(result4, date(2015, 8, 31))
+        self.assertEquals(result5, date(2015, 11, 30))
 
     def test_one_year(self):
-        result = relative_date_to_date('1y')
-        self.assertEquals(result, date.today() + timedelta(365))
+        test_date = date(2015, 1, 10)
+        result = relative_date_to_date('1y', test_date)
+        self.assertEquals(result, date(2016, 1, 10))
+
+    def test_leap_year(self):
+        test_date = date(2016, 2, 29)
+        result1 = relative_date_to_date('1y', test_date)
+        result2 = relative_date_to_date('4y', test_date)
+        self.assertEquals(result1, date(2017, 2, 28))
+        self.assertEquals(result2, date(2020, 2, 29))
 
     def test_zero_months(self):
         result = relative_date_to_date('0m')