Add 6 hours per year converting from interval

This is the same operation performed by postgres when extracting the
number of seconds of an interval:

    =# select extract(epoch from '1 year'::interval) / 60 / 60 / 24;
     ?column?
    ----------
       365.25

This way `extract(epoch from interval)` will match the resulting
`timedelta.total_seconds()`.

Close #570

psycopg/typecast_datetime.c

@@ -448,6 +448,9 @@ typecast_PYINTERVAL_cast(const char *str, Py_ssize_t len, PyObject *curs)
     /* add the days, months years together - they already include a sign */
     days += 30 * (PY_LONG_LONG)months + 365 * (PY_LONG_LONG)years;
 
+    /* Postgres adds 6 hours for each year, or for each 12 months */
+    seconds += 6 * 60 * 60 * (PY_LONG_LONG)(years + (months / 12));
+
     return PyObject_CallFunction((PyObject*)PyDateTimeAPI->DeltaType, "LLl",
                                  days, seconds, micros);
 }

tests/test_dates.py

@@ -223,6 +223,32 @@ def test_parse_interval(self):
     def test_parse_negative_interval(self):
         self._check_interval('-42 days -12:34:56.123456')
 
+    def test_parse_mixied_signs_interval(self):
+        # Intervals in postgres have 4 blocks
+        # -1 year -2 mons +3 days -04:05:06
+        # each can be missing, present with no sign, positive, negative
+        # test all the combos
+        def cases(s):
+            return ['', s, '-' + s, '+' + s]
+
+        from itertools import product
+        for parts in product(
+                cases('1 year'), cases('2 months'),
+                cases('3 days'), cases('04:05:06')):
+            s = ' '.join(parts)
+            if s.isspace():
+                continue
+            self._check_interval(s)
+
+    def test_parse_interval_element(self):
+        for i in range(-1000, 1100, 100):
+            self._check_interval('%s years' % i)
+            self._check_interval('%s months' % i)
+            self._check_interval('%s days' % i)
+            self._check_interval('%s hours' % i)
+            self._check_interval('%s minutes' % i)
+            self._check_interval('%s seconds' % i)
+
     def test_parse_infinity(self):
         value = self.DATETIME('-infinity', self.curs)
         self.assertEqual(str(value), '0001-01-01 00:00:00')