added workaround for Matrix3D not allowing scaling values of zero
avoiding exception when querying bounds of 3D objects that are not part of the display list

If the objects are transformed in any way, the value will not be accurate; but it's better than throwing an exception. Note that the problem not only affected Sprite3D objects, but an 2D objects in a 3D space.
fixed argument type in 'transformMatrix3D' method
added information about default value for 'supportHighResolutions' to API reference
changed the 'this' pointer within tween callbacks to point to the tween instance (closes #632)
now normalizing 'rotationX' and 'rotationY' in Sprite3D; added 'MathUtil.normalizeAngle'
optimization: Sprite3D objects without 3D transformation components are now batched

In other words, Sprite3D objects with zero values for "z", "rotationX" and "rotationY".
updated Sprite3D API docs
added support for pivoting to Sprite3D
updated AIR xmls for v15
changed samples to always enable 'handleLostContext'
added Event.FATAL_ERROR, dispatched from the Starling instance on fatal errors (closes #631)
now preventing all further rendering in 'stopWithFatalError (refs #631)
fixed 2d transformation matrix setter of Sprite3D

When setting a 2D transformation matrix, all 3D transformation components must become zero.
added some tests for MatrixUtil
added MathUtil test
added first tests for Sprite3D
fix travis-ci build - set depth to 10 to prevent fatal error
Merge pull request #634 from honzabrecka/fix-travis-ci

fix travis-ci build (changed depth from 1 to 10)
fixed small assignment error