Unicode handling for column addressing

Hi,

addressing a column ct[col].rootdir with a unicode col string, generated an error like this: 

    /srv/python/venv/local/lib/python2.7/site-packages/bcolz/ctable.pyc in __getitem__(self, key)
       1098         # All the rest not implemented
       1099         else:
    -> 1100             raise NotImplementedError("key not supported: %s" % repr(key))
       1101 
       1102         # From now on, will only deal with [start:stop:step] slices

    NotImplementedError: key not supported: u'o'

This solves that.