Closes #128; Add Layer Toggling

This adds the ability to show/hide layers on demand to the basic and real-time plots. Introduces few new methods available to every chart:

* `showLayer(labelOrIndex)` - Given a name or index shows a given layer
* `hideLayer(labelOrIndex)` - Given a name or index hides a given layer
* `toggleLayer(labelOrIndex)` - Toggles a layer's visibility state
* `isLayerVisible(labelOrIndex)` - True if a layer is visible, false otherwise
* `getVisibleLayers()` - Returns an array containing only the visible layers in a plot

To test this new functionality use the Render tests (located in `tests/render`) as well as run the unit tests.