add Scale.ModifiedLog

added unit tests to scale.ts, and quicktests
to ModifiedLogScale_test.js
ticks don't trail off the end
ModifiedLogScale._niceDomain = id
better tick generation for ModifiedLog

It's complicated, but looks good.

It puts ticks in "clusters" that get more and more visually
dense, just like regular log scales. Values between
-pivot and pivot have ticks drawn linearly.

Ticks are partitioned between negative, linear, and positive
based on how many pixel each will take up.
prefer powers of 10, add extra clusters if possible
increasing, continous tests improvements
this.pivot = this.base
ModifiedLog.showIntermediateTicks() added

Defaults to false: only [10, 100, 1000, ...] is shown
by default.
ticks between pivots respect showIntermediateTicks
tests closeTo() Math.log()
Merge pull request #710 from palantir/modified-log-scale

add Scale.ModifiedLog
Release version 0.21.0
Merge branch 'master' into multi-time-axis