Fixes #30 - Basic Plot "Label-less" Layers

Fixes an issue where multi-series basic plots would not render correctly unless each of the layers was provided with a label. Simplified the process by simply keeping a reference to the categorical index on each layer object when setting initial data for a plot (see `Epoch.Chart.Base.setData`). To fix each of the plots we simply use the category index as opposed to label when grouping data in a series with d3.

For testing I have added a set of new "Label-less layer tests" to each of the basic rendering test pages. To view them simply open `test/index.html` in a local browser and navigate to each individual chart page.
This and line 46 will replace the label name with the category number. If you'd like to hide labels on the plot completely this can be achieved via CSS by setting the text element display to none.