Fix support for datetime axes in 2d plots.

This PR adds full support for datetime coordinates to `pcolor`, `pcolormesh` and `contour`/`contourf`.

Currently one can make a contour plot of a data set that uses datetime objects as one of its coordinates, provided that the coordinates are 1d. For 2d coordinates an unintelligible error was produced. For `pcolor`/`pcolormesh` an error about not being able to convert a value to float was raised regardless of whether the datetime coordinate was 1d or 2d.

This PR fixes these issues by making sure that coordinate values are passed through `Axes.convert_xunits` or`Axes.convert_yunits` before being used to construct the plot, and making sure that the call will do the conversion by ensuring the coordinates are 1d when passed to the converter.

I haven't added anything to whats_new.rst or CHANGELOG yet, I'm still not sure if this is a new feature or a bug fix!