minor bug of transpose?

Hi,

I experienced the following behaviour of transpose operator:

```
import dynet as dy
mod = dy.Model()
px = mod.add_parameters((128,1))
ix = dy.parameter(px)
ix_mean = dy.mean_dim(ix, {0}, True)
ix_mean_t = dy.transpose(ix_mean)
```

I got the following error:
```
Traceback (most recent call last):
File "", line 1, in
File "_dynet.pyx", line 3124, in _dynet.transpose
File "_dynet.pyx", line 3139, in _dynet.transpose
ValueError: Dimensions passed to transpose (2) must be equal to dimensions in input tensor (1)
```

I understand that ix_mean in this case has dimension (1,1) and transpose should work (though it does nothing).

I think the dim_forward function of transpose operator should be corrected as follows:

```
Dim Transpose::dim_forward(const vector& xs) const {
DYNET_ARG_CHECK(xs.size() == 1, "Bad arguments to Transpose: " << xs);
DYNET_ARG_CHECK(xs[0].nd == dims.size() || xs[0].num_nonone_dims() == 1 || xs[0].nd == 1, "Dimensions passed to transpose (" << dims.size() << ") must be equal to dimensions in input tensor (" << xs[0].nd << ')');
Dim ret(xs[0]);
ret.nd = dims.size();
for(size_t i = 0; i < dims.size(); ++i)
ret.d[i] = xs[0][dims[i]];
return ret;
}
```

How do you think? I will do a PR? Thanks!