Use an unambiguous format for dates.

Use month abbreviations instead of numeric months, since day-of-week
abbreviation is already locale-dependent anyway.  Putting the day-of-month
before the month abbreviation avoids the need for a comma after the
day-of-week.

Resolves monkbroc#5.

lib/slack_time.js

@@ -212,8 +212,8 @@ function currentUserInfo(everybody, userId) {
   }
 }
 
-function formatMonthDay(dateMoment) {
-  return dateMoment.format("MM-DD");
+function formatDayAndMonth(dateMoment) {
+  return dateMoment.format("D MMM");
 }
 
 function formatDayOfWeek(dateMoment) {
@@ -233,7 +233,7 @@ function abbreviate(string) {
 }
 
 function formatSlackDate(dateMoment) {
-  return '_' + formatDayOfWeek(dateMoment) + ' ' + formatMonthDay(dateMoment) + '_ ';
+  return '_' + formatDayOfWeek(dateMoment) + ' ' + formatDayAndMonth(dateMoment) + '_ ';
 }
 
 function formatSlackTime(dateMoment) {