Pinch zoom ux

[Alpt]

This pull request is an experimentation on improving the UX of pinch-zoom.
It starts from pull request 990, which fixes two functional bugs, and it adds the following:

• do not go out of the graph extremes (g.xAxisExtremes()), both during pan and during zoom. Use g.getNumericOption("panEdgeFraction") to have a tolerance about this. Perhaps, this fixes 713.
• sensitiveness dampening: smaller pinches count much less for scaling
• context.pinchOutOfExtremes: do not pinch-zoom if the center of the pinch is out of the extremes of the axes
• There was this feature: "make pinches in a 45-degree swath around either axis 1-dimensional zooms". I found it needed improvement, so I disabled it. Without it, I find the graph to be very usable, perhaps thanks to the dampness of sensitiveness. But perhaps, you want to enable it. It is easy: just de-comment two lines.

[coveralls]

Coverage decreased (-0.8%) to 89.354% when pulling 87f18e3 on Alpt:pinch-zoom-ux into e9155b2 on danvk:master.