Two new options: "inset" and "fromCenter"

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.