Dataset.__repr__ should show array values

[shoyer]

This would be much more useful than the grid of coordinate dimensions we show now -- though we could keep that under another alias (e.g. 'dim_info()')

Something like the following would be great (inspired by R's str function):

<xray.Dataset>
Dimensions:    (time: 500, location: 57, realization: 10)
Coordinate indexes:
  time         2000-01-01 2000-01-02 2003-12-31 ...
  location     'california 'oregon' 'washington' 'idaho' ...
  realization  0 1 2 3 4 5 6 7 ... 
Other coordinates:
  latitude     (location,) 43.0 43.5 44.0 44.5 45.0 ...
  longitude    (location,) -90 -89 -88 -87 -86 -85 ...
  ref_time     () 1999-12-31
Variables:
  temperature  (time, location, realization) 1.22 2.35 3.22 -2.33 ...
  tmin         (location, realization) -3.34 2.52 3.53 5.33 4.23 ...
Attributes:
  Empty