Fixed a couple of bugs related to the angle widget.

Congruent match option wasn't always working for two different reasons:
1) KhanUtil.findAngle can return negative values if used with 3 points.
2) The topmost default ray control point was being snapped to the grid,
   and didn't respect the snapDegrees option.

Test Plan:
Created an item requiring a 60 degree angle (congruent matching), verified
that no 60 degree angle failed to match.

Auditors: jack