Issue 48 - hacking around the missing pointer-events:none in internet ex...

This is a fix to avoid the flickering of tooltips in pie charts in internet explorer 10 (https://github.com/novus/nvd3/issues/48). The base idea came from the https://github.com/kmewhort/pointer_events_polyfill repository. On leaving the pie I proof if the mouse is on the tooltip or not. If it is on the tooltip, then I prevents its hiding. See the flowcharts in the bug description. I tried to document my changes in the code.