jonathantompson: added cuda support for nn.Abs (and added test for it).

pushing latest changes from torch7-distro.

Not pushing lbouttou's commit https://github.com/torch/torch7-distro/commit/8bfc8de486120015aad1f9a06647e1f2137386af#diff-fbdab6b8d329385b8ba089bd36f6be1d
, because seems like the line-endings were changed and the diffs are a mess.