torch.remap(t1, t2, f)

This PR adds torch.remap, a function to recursively traverse two tables of tensors and apply a function to these. This could replace some of the local functions used in nn table layers, and could help with unit testing these. Includes unit tests and doc. 

I also moved torch.isTensor to Tensor.lua in another commit as I wasn't able to access it otherwise, and I believe that is where it should be.

I am open to any name changes.