Fix behaviour of torch.isTensor

In my understanding, torch.isTensor should only return true if the tested object is an actual Tensor, not if it is the table torch.Tensor (or torch.FloatTensor, etc)

That means:
t = torch.FloatTensor 
torch.isTensor(t) -- should be false
torch.isTensor(t()) -- should be true
Merge pull request #100 from GeorgOstrovski/patch-2

Fix behaviour of torch.isTensor