Add TimeScale

D3.Scale.TimeScale and D3.Scale.QuantitiveScale basically have the same functionality,
due to the "numeric-ness" of Javascript's Date object, but weren't compatible because
Typescript considers the signatures incompatible. However, calls such as

var scale = new Plottable.QuantitiveScale(d3.time.scale());

have worked in pure Javascript. To that end, Plottable.Timescale uses some casting
to convince Typescript that it will be okay...
Simplified casting.
Merge pull request #359 from palantir/time-scale

Add TimeScale