Have BaseAxis take in `any` as the formatter arg.
If it receives a function, then it wraps it in a CustomFormatter
CustomFormatter is set properly if generated thru the Axis.formatter
Merge pull request #679 from palantir/cat-axis-formatter-2

Have BaseAxis take in `any` as the formatter arg.
Merge branch 'master' into HEAD

Conflicts:
	plottable.d.ts
Merge pull request #686 from palantir/customFormatter-api

Custom formatter api