duration may have extra '+' in value

from rfc5545 spec:
 dur-value = (["+"] / "-") "P" (dur-date / dur-time / dur-week)

Signed-off-by: Hiroaki KAWAI <kawai@stratosphere.co.jp>
Merge pull request #149 from iHiroakiKawai/duration_format

duration may have extra '+' in value