Enable sorting of numerical columns

Columns (float or integer type) may be sorted in ascending or descending
order, by clicking arrows next to the header titles.

Additionally, CSS rules were changed and event listeners removed for
columns which cannot reasonable be charted (i.e. you can only chart
columns with a number type: gone are the days of weird histograms of
booleans or strings).

This involves moving records from props to state.

![](http://cl.ly/image/1n013m3J1r1S/Screen%20Shot%202014-09-26%20at%201.15.47%20PM.png)

fixes #71
What's the `prop` in this var name referring to? Maybe `aClasses`? or `headerClasses`?