Refs #128; Added layer visibility flags to the Base chart.
Refs #128; Basic charts now show and hide layers appropriately.
Refs #128; Added event tests for hiding and showing layers.
Refs #128; Got show/hide working for real-time area plots (without animation).
Refs #128; Real-time stack plot now handles queued entries when toggling layers.
Refs #128; Bar plot now toggles layers.
Refs #128; Real-time line charts now toggle layers.
Refs #128; Heatmaps now toggle layers.
Merge pull request #129 from fastly/rsandor/show-hide-layers

Closes #128; Add Layer Toggling