Added keys in Contour, Heatmap, Histogram2d, Histogram2dContour,
ColorBar, Contours.

I took for reference the keys that can be modified in the web GUI.

Some key ordering were modify.
Descriptions were added.

Could be better-optimized with more shortcuts.
drop_in shortcuts for:

zauto, zmin, zmax, zsmooth, autocontour, ncontours, contours,
x0, dx, y0, dy, xtype, ytype

with default values and improved descriptions.
Merge pull request #3 from etpinard/2d_objs_keys

Added keys in Contour, Heatmap, Histogram2d, Histogram2dContour, ColorBar & Contours.