Adding option of having a shadow around tooltip

When Chart-level option "tooltipShadowColor" is populated with color
code, adds the shadow.  shadowBlur = 4 is used.
Show non-numeric values as labels for tooltips

This allows to pass in non-numeric values like "N/A", "Not Reported",
etc. Chart.js will process those the same it does null values (not
plotting them) and still show the appropriate value in tooltips
Option to only display points within cursor area

By setting "tooltipCursorSensitive" to true, only the data points within
"tooltipCursorHitPixels" will be displayed in the tooltip box. Except
for data points with a "custom value" (those not numeric, not null),
those will display anywhere on the y-axis, same as current default.
Also, a minor fix to account for the height of the blurry pixels (when
using shadow)  when determining that the tooltip box is bleeding outside
of the canvas.
Moved settings to Core

They should have been there in the first place
Merge remote-tracking branch 'origin/feature/tooltips-improvements'
Small change brought up by JSHint validation
Merge remote-tracking branch 'origin/feature/tooltips-improvements'