allow habit-style repeaters in timestamps

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.

Quoted from:

  http://orgmode.org/manual/Tracking-your-habits.html
suppress blank line added between drawers in $doc->as_string()

When calling $doc->as_string(), an extra blank line is generated between drawers.  This change prevents the extra blank line.
Merge pull request #3 from wu/master

allow habit-style repeaters in timestamps