Parallelizing most common for loops (TensorMath).

Ideally this should be done withing the apply/map macros,
        but for some reason it's not trivial.

For now I've provided a macro called for_omp, which provides
a parallel for loop. The variable used as an iterator must
be declared within the for loop scope:
for_omp(long i=0; ...) {
Merge branch 'master' of github.com:andresy/torch
Merge branch 'master' into luarocks