stripped string after since to remove leading spaces

Test added. There is one (unrelated ? error) in tst_netcdftime. 

```
======================================================================
ERROR: test_select_nc (__main__.TestDate2index)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "test/tst_netcdftime.py", line 387, in test_select_nc
    millisecs = int(date2num(d,unix_epoch,calendar='proleptic_gregorian'))
  File "utils.pyx", line 124, in netCDF4.date2num (netCDF4.c:4098)
    basedate = _dateparse(units)
  File "utils.pyx", line 21, in netCDF4._dateparse (netCDF4.c:2901)
    basedate = dparse.parse(isostring)
  File "/usr/local/Python271_so/lib/python2.7/site-packages/dateutil/parser.py", line 698, in parse
    return DEFAULTPARSER.parse(timestr, **kwargs)
  File "/usr/local/Python271_so/lib/python2.7/site-packages/dateutil/parser.py", line 302, in parse
    res = self._parse(timestr, **kwargs)
  File "/usr/local/Python271_so/lib/python2.7/site-packages/dateutil/parser.py", line 350, in _parse
    l = _timelex.split(timestr)
  File "/usr/local/Python271_so/lib/python2.7/site-packages/dateutil/parser.py", line 144, in split
    return list(cls(s))
  File "/usr/local/Python271_so/lib/python2.7/site-packages/dateutil/parser.py", line 44, in __init__
    instream = StringIO(instream)
TypeError: initial_value must be unicode or None, not str
```