removing reliance on a global table

As you mentioned in your unhappy comment here: https://github.com/jucor/torch-randomkit/pull/42

You didn't want to hack your requires into a global table.
The way around it is keeping a local table that gets cached by require itself.

I did not fully test this branch because you still have references to torchffi and we cant use torchffi anymore, it segfaults, because of conflicts with the torch7 native :data/:cdata ffi functions.