Retrieve the intersection ray/voxel after castRay.
Changed function name an solved instability.

The function now returns whether or not an intersection has been
found.

Solved some numerical instabilities with the planes boundaries. The
planes are enlarged by 1e-6 on each side so that point falling on the
intersection (up to numerical stability with substract/division of
double) are included.
Added an example of how to use getRayIntersection
Merge pull request #49 from fmder/intersect

Retrieve the intersection ray/voxel after castRay using "getRayIntersection"