Dataset Metadata, Better Axes, More Powerful Accessors

• IDataset now includes an IMetadata field, which includes the css class, color, and any other data that might be series-wide (bar renderer bar width, etc).
• In conjunction with the previous change, dataset accessors (IAccessors) are now more powerful, taking in a single datum, the datum's index, and also dataset-wide metadata. This allows, for example, the use of color scales to set series colors.
• Renderers now also take in a colorAccessor, which can be used to vary the color of rendered elements (for example, to implement color scales on a scatterplot).
• Axis now hides tick labels that would land outside the bounding box, rather than cutting them off.
• Axis also now has the ability to position tick labels relative to the tick mark (left/right/center for XAxis and top/middle/bottom for YAxis)
• plottable.d.ts no longer shows private variables. Previously, they were shown with an implicit type.