SelectTable can resize

depends on https://github.com/torch/cutorch/pull/33

fixes #50

```lua
~/projects/nn$ th
> require 'nn'
true	
                                                                      [0.0150s]	
> st = nn.SelectTable(1)
                                                                      [0.0001s]	
> input = {torch.rand(1)}
                                                                      [0.0001s]	
> gradOutput = torch.rand(1)
                                                                      [0.0001s]	
> st:forward(input)
 0.7282
[torch.DoubleTensor of dimension 1]

                                                                      [0.0001s]	
> print(st:backward(input, gradOutput))
{
  1 : DoubleTensor - size: 1
}
                                                                      [0.0001s]	
> input = {torch.rand(1), torch.rand(1)}
                                                                      [0.0001s]	
> print(st:backward(input, gradOutput))
{
  1 : DoubleTensor - size: 1
  2 : DoubleTensor - size: 1
}
                                                                      [0.0003s]	
> input = {torch.rand(1), torch.rand(1,2), torch.rand(1)}
                                                                      [0.0003s]	
> print(st:backward(input, gradOutput))
{
  1 : DoubleTensor - size: 1
  2 : DoubleTensor - size: 1x2
  3 : DoubleTensor - size: 1
}
```
 
So now it works for mutable sizes.