Major PrawnCharts revamping

Proper scaling for Columns and Axes, negative values properly accounted for, LayerContainer helpers,
super cool significant digits rounding helper, beginning of idea of relevant layer rendering for Components,
documentation, code cleanup, few other small thing I may have forgotten…
Still very much a work in progress
New significant digit helper

Abstracted out some of the code needed to determine how many significant digits
to round a number to and put it in a new helper.
Merge pull request #11 from singjsong/develop

Major PrawnCharts revamping