CSV with timedeltas generated with and old version of pandas cannot be parsed anymore
I have been using an older version of pandas (0.13.1) and serialized several timedeltas in a data frame. These timedeltas ended up like this in the generated CSV file:

```
-1 days +23:30:00.000000000
```

This format cannot be parsed anymore. It seems that the + sign in front of the hours causes the parsing to fail:

```
In [24]: pd.to_timedelta('-1 days 23:30:00.000000000')
Out[24]: numpy.timedelta64(-171000000000000,'ns')

In [25]: pd.to_timedelta('-1 days +23:30:00.000000000')
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-25-79414e44ea89> in <module>()
----> 1 pd.to_timedelta('-1 days +23:30:00.000000000')

/usr/lib/python2.7/dist-packages/pandas/tseries/timedeltas.pyc in to_timedelta(arg, box, unit)
     60 
     61     # ...so it must be a scalar value. Return scalar.
---> 62     return _coerce_scalar_to_timedelta_type(arg, unit=unit)
     63 
     64 _short_search = re.compile(

/usr/lib/python2.7/dist-packages/pandas/tseries/timedeltas.pyc in _coerce_scalar_to_timedelta_type(r, unit)
     76 
     77         # we are already converting to nanoseconds
---> 78         converter = _get_string_converter(r, unit=unit)
     79         r = converter()
     80         unit='ns'

/usr/lib/python2.7/dist-packages/pandas/tseries/timedeltas.pyc in _get_string_converter(r, unit)
    140 
    141     # no converter
--> 142     raise ValueError("cannot create timedelta string converter")
    143 
    144 def _possibly_cast_to_timedelta(value, coerce=True):

ValueError: cannot create timedelta string converter
```
