Fixes GH1199

Previously (on master):

    <xarray.Dataset>
    Dimensions:  (x: 1, y: 2)
    Coordinates:
      o x        (x) -
      o y        (y) -
    Data variables:
        foo      (x, y) int64 1 2

Now:

    <xarray.Dataset>
    Dimensions:  (x: 1, y: 2)
    Coordinates:
        *empty*
    Unindexed dimensions:
        x, y
    Data variables:
        foo      (x, y) int64 1 2

This version of the `__repr__` should be much more self-explanatory.