bump version number to 1.5.1

Changelog

@@ -1,4 +1,4 @@
-version x.x.x (release tag vx.x.x.rel)
+version 1.5.1 (not yet released)
 ======================================
  * added support for "common_year" and "common_years" units for "noleap" 
    and "365_day" calendars (issue #5, PR #246)

src/cftime/_cftime.pyx

@@ -38,7 +38,7 @@ cdef int[12] _dayspermonth_leap = [31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 3
 cdef int[13] _cumdayspermonth = [0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334, 365]
 cdef int[13] _cumdayspermonth_leap = [0, 31, 60, 91, 121, 152, 182, 213, 244, 274, 305, 335, 366]
 
-__version__ = '1.5.0'
+__version__ = '1.5.1'
 
 # Adapted from http://delete.me.uk/2005/03/iso8601.html
 # Note: This regex ensures that all ISO8601 timezone formats are accepted - but, due to legacy support for other timestrings, not all incorrect formats can be rejected.
@@ -145,7 +145,8 @@ def date2num(dates,units,calendar=None,has_year_zero=None):
     **units**: a string of the form **<time units> since <reference time>**
     describing the time units. **<time units>** can be days, hours, minutes,
     seconds, milliseconds or microseconds. **<reference time>** is the time
-    origin. **months_since** is allowed *only* for the **360_day** calendar.
+    origin. **months since** is allowed *only* for the **360_day** calendar
+    and **common_years since** is allowed *only* for the **365_day** calendar.
 
     **calendar**: describes the calendar to be used in the time calculations.
     All the values currently defined in the
@@ -439,7 +440,8 @@ def num2date(
     **units**: a string of the form **<time units> since <reference time>**
     describing the time units. **<time units>** can be days, hours, minutes,
     seconds, milliseconds or microseconds. **<reference time>** is the time
-    origin. **months_since** is allowed *only* for the **360_day** calendar.
+    origin. **months since** is allowed *only* for the **360_day** calendar
+    and **common_years since** is allowed *only* for the **365_day** calendar.
 
     **calendar**: describes the calendar used in the time calculations.
     All the values currently defined in the