Handle more gracefully the case where a texture
directive has no image specified.  Prior to this
it was throwing an ArrayIndexOutOfBounds exception
which was silently swallowed even though the
import fails.
Bugfix: fixed a bug that caused ugly artifacts to appear sometimes while
computing constraints. The fix also improved the constraints computation
speed at least several times :)
Merge pull request #35 from jMonkeyEngine/master

goodies