New Dataset.__repr__

@toddsmall and @hdail both reported surprise that the order of dimensions as
printed in a Dataset's string representation are not necessarily the same as
the order of any variable dimensions.

This patch alters `Dataset.__repr__` so the order of the dimensions for each
variable is now printed. Hopefully this should make things clearer.

Example of the new look:

    <xray.Dataset>
    Dimensions:     (dim1: 100, dim2: 50, dim3: 10, time: 20)
    Coordinates:
        dim1             X
        dim2                        X
        dim3                                  X
        time                                            X
    Noncoordinates:
        var1             0          1
        var2             0          1
        var3             1                    0
    Attributes:
        Empty

Vs. the old look:

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

I added Coordinates to make it clear that these are items in the Dataset.
I removed the dashes '-' because they added visual noise that made it harder
to read the axis numbers.

Note: the commits for this PR are applied on top of those for #83, so
they can be merged sequentially without the need to rebase.