Add in formatters.

Plottable.Util.Formatters now includes functions that generate formatters:

- identity() : String-ifies the input.
- general() : Shows at most [precision] decimal places (using round-half-up).
- fixed() : Shows exactly [precision] decimal places (IEEE 754 rounding).
- currency() : Displays fixed() values with a currency symbol (defaults to $)
- percentage() : Dipplayes fixed() values as percentages.