[Issue]
  Text doesn't rotate correctly.

[Cause]
  In benri.geometry.Matrix2D, Math.atan() is used for obtaining the rotation angle.
  However, its return value ranges from -Math.PI/2 to Math.PI/2
  So, it is necessary to scale the value to get the full 0-360 degrees.

[Fix]
  Changed Matrix2D.getRotation() so that we can obtain the scaled radians.
Reworked based on the feedback.
Merge pull request #4 from kuu/master

[Bugfix] Text doesn't rotate correctly.