Update nvd3 d3 dependency to ~, so no conflicts with packages who also require d3 at a newer 3.3 version.
Merge pull request #376 from DavidSouther/d3_dependency_update

Update nvd3 d3 dependency to ~
Made a fix to calcApproxTextWidth, to fix bug on IE9.
ran grunt production to make nv.d3.min.css. Also tagging for version 1.1.15b
updated README
added stackedAreaPlusLineChart, still some bugs to be worked out