Remove the `Dataset.T` property?

[shoyer]

The fact that ds.T transposes a dataset rather than returning ds['T'] is a repeated point of confusion:
http://stackoverflow.com/questions/41879269/xarray-dataarray-opens-as-a-dataset/41885639#41885639
http://stackoverflow.com/questions/40270929/how-to-read-a-variable-called-t-with-xarray