Incrementing from middle of month should result in datetime instance
on same date:

>>> from xix.utils.timetool import incrementMonth
>>> from time import strptime, strftime, mktime, tzset
>>> from datetime import datetime
>>> t = strptime('2005-01-15', '%Y-%m-%d')
>>> dt = datetime.fromtimestamp(mktime(t))
>>> dt2 = incrementMonth(dt)
>>> print strftime('%Y-%m-%d', dt2.timetuple())
2005-02-15
>>> dt2 = incrementMonth(dt, 3)
>>> print strftime('%Y-%m-%d', dt2.timetuple())
2005-04-15
>>> dt2 = incrementMonth(dt, -1)
>>> print strftime('%Y-%m-%d', dt2.timetuple())
2004-12-15
>>> dt2 = incrementMonth(dt, 12)
>>> print strftime('%Y-%m-%d', dt2.timetuple())
2006-01-15


