allow habit-style repeaters in timestamps

This change allows habit-style repeaters in timestamps.  Currently Org-Parser dies when it encounters such timestamps.  Here is a quote from the docs describing the timestamp format:

  http://orgmode.org/manual/Tracking-your-habits.html

The TODO may also have minimum and maximum ranges specified by using the syntax ‘.+2d/3d’, which says that you want to do the task at least every three days, but at most every two days.