Fix performance regression for decode_cf_variable

We were passing the netCDF4 Variable directly to decode_cf_datetime, which
does a very expensive np.asarray() call (fixed in the master branch of
netCDF4-python). By passing the data as a numpy array, this call is *much*
faster.
Merge pull request #61 from akleeman/decode-cf-performance

Fix performance regression for decode_cf_variable