refs #153 : add transitions

* transitions active again when path enters or exits chart
Merge branch 'stackedarea' of https://github.com/MikeRoetgers/nvd3 into development

Conflicts:
	src/models/stackedArea.js
Updating stackedArea.js to use d3 version 3 transition chaining syntax.
Adding transitionDuration option to stackedArea.
Adding transition to StackedAreaChart example.
Rebuilding nv.d3.js. Bumping version to 1.1.1b, due to stackedArea changes.