Fixing chart.unlayer (removing old refernece to internal layer array) and adding chart.relayer. Added tests to both.
Addressing @jugglinmike's comments: Moving tests to integration.js, removing _chart reference from layer, removing 'relayer' in favor of layering.
removing function definition test
Adding verification that upon layer reattaching, the second argument is in fact a layer.
Release v0.1.3