Stay up-to-date on torch's thread-safization

@soumith's pull-request torch/torch7-distro#206 is intending to make torch thread-safe, in collaboration with @koraykv, including the RNG.

This issue is a reminder to keep a close eye on it to make sure that `torch-randomkit` complies with this thread-safe behaviour :)

The C part for randomkit is thread-safe, thanks to its `state` structure which comes as the first argument of all the C functions, but the way we create a single shared state in the lua code and pass it automatically as the first argument is _not_ thread-safe.