Added exportToOFF(), perturbed duplicate vertices, and modified Range() to be v0.3 compatible

I've added a couple of new functions, exportToOFF(), and clean().   I'm not sure why, but the meshes I create with isosurface() generally have some zero-area faces.   clean() removes all zero-area faces and duplicate vertices.  If the isosurface() can be made to do the same thing, perhaps that is better than a post-cleaning step.
I was also trying to run my code against the latest 0.3 Julia, and there was one error with Range() that was blocking.  I changed it so it seems to work in both 0.2 and 0.3.