changes in tick locator, contour level selection

All,

CVS now has some changes that need testing:

1) ticker.AutoLocator is using a new MaxNLocator originating in a suggested algorithm by Rob Knight. I don't expect it to do *exactly* what the original AutoLocator does, but it should be similar, and about as good. If so, then in the future it will facilitate better control of the number of ticks on an axis. Please try it with your most pathological axis examples as well as normal use cases. You can test it with any plot command, because AutoLocator is used by default.

2) The MaxNLocator is now used in contour to select the contour levels when they are not given explicitly. Previously the levels would be linearly distributed between the data extremes; now they land on nice values, like "0.0" instead of "0.1379".

Eric