Axis.Category._requestSpace caches character sizes
indentation
only clear chr2Measure if a character changed size
typo: computTickWH -> computeTickWH
moved caching code into Util.Text.CachingMeasurer
Add LargeNumber Formatter.

Formats large numbers to short strings. Ex:

1000000 --> "1.00M"
1234567890 --> "1.23G"

Close #577.
comments, CANONICAL_CHR used instead of "x"
Util.Cache created and now used instead of MeasureCache
Have AreaPlot add a padding exception at y=0.
plot.remove() causes scale to resize
quicktest fix up: setTimeout() removed
arrayEq handles null and undefined
Cache uses d3.map instead of regular object
valueEq defaults to ===
Util.Text.CachingCharacterMeasurer added
Instead of adding a constant exception at 0, use extent calculation to detect if there is a constant baseline.
If the baseline is constant, add an exception.
Add unit tests
cacheTests.ts added
cachingCharacterMeasurerTests.ts added
Update the areaPlot so baseline exception changes if data changes.
Add additional unit test.
style / useless code removal