Improved __repr__/__str__ for xray objects

The new `__repr__` for xray.Dataset is inspired by the representation
for iris.Cube:

    <xray.Dataset>
    Coordinates:     (time: 20, dim1: 100, dim2: 50, dim3: 10)
    Non-coordinates:
        var1              -         X          X         -
        var2              -         X          X         -
        var3              -         X          -         X
    Attributes:
        Empty

The new `__repr__` for xray.XArray and xray.DatasetArray shows the actual data
as summarized by `repr(array.data)`, as long as the data is an ndarray or
has fewer than 10^5 elements (~400 KB):

    <xray.DatasetArray 'my_variable' (time: 2, x: 3)>
    array([[1, 2, 3],
           [4, 5, 6]])
    Attributes:
        foo: bar

`__repr__` not showing the data was a complaint I heard about the prior
representation.

I removed the separate `__str__` implementation so we can have one canonical
string representation (both implementations showed equivalent information).

I am definitely open to suggestions for improving either of these!