Two new options: "inset" and "fromCenter"

[nrabinowitz]

This implements two new options for the powerTip:

• inset determines the distance between the arrow and the corner in ne, nw, se, and sw placements (formerly hard-coded to 20px)
• fromCenter allows tip placement to be pulled closer to the center of the target element's bounding box. Setting fromCenter to 0 puts the tip directly in the center; 1 places it at the edge (the current behavior); and anything in between will place the tip a percentage between the center and the edge.