Renamed variables (fix issue #6)

[sebastianv89]

The result from ge_scalarmult_base is in projective coordinates, so the variable names should reflect this: I replaced one with ed_z. Variable ed_y was not used, so I removed it. (see also this answer and the curves mailing list)

[moxie0]

Thanks but this isn't a functional change, so we're going to leave it as-is for now.