adding torch.isSameSizeAs

A small function to check if two tensors are of the same order.

doc + unit test included.

```lua
 > x = torch.Tensor(4,5)
 > y = torch.Tensor(4,5)
 > = x:isSameSizeAs(y)
 true
 > y = torch.Tensor(4, 6)
 > = x:isSameSizeAs(y)
 false
> y = torch.Tensor(1)
 > = x:isSameSizeAs(y)
 false
 ```