Silence iso8601 debug messages in verbose mode

In verbose mode, iso8601 package produces the following useless debug
messages:

DEBUG: iso8601.iso8601 Got u'2014' for 'year' with default None
DEBUG: iso8601.iso8601 Got u'09' for 'monthdash' with default 1
DEBUG: iso8601.iso8601 Got 9 for 'month' with default 9
DEBUG: iso8601.iso8601 Got u'14' for 'daydash' with default 1
DEBUG: iso8601.iso8601 Got 14 for 'day' with default 14
DEBUG: iso8601.iso8601 Got u'16' for 'hour' with default None
DEBUG: iso8601.iso8601 Got u'31' for 'minute' with default None
DEBUG: iso8601.iso8601 Got u'23' for 'second' with default None

This change silences them.

Change-Id: Ibd0c5bb48b1ab86aaee2c8cb4f27b7e5fa8b4bef
Merge "Silence iso8601 debug messages in verbose mode"