[STYLING] Add in new colors, change axis spacings.

Specifically:
  - Bring `Axis` styles more in line with Design's choices.
  - Add in `Colors` class to store (as static values) canonical Plottable colors.
  - Add in new 10-color scale. Creating a new color scale with
    `new Plottable.Scale.Color()` will use the new scale instead of
    creating an empty array.
  - Default plot color has been changed to `Plottable.Core.Colors.INDIGO`.
  - Axis tick labels are now spaced farther from the tick marks
    by default to improve readability.
  - `Interaction.DragBox` now draws in the background of its `Component`.

Close #805.