mlab.imshow positions the image incorrectly

With mayavi 4.5.0, setting the `extent` of an image while calling `imshow` does not cause an exception like it did before (see issue #37 ). But still, specifying the `extent` keyword does not have the expected effect. In the following example, the image is incorrectly placed in the xy-plane, instead of the xz-plane as specified. The scaling is also wrong.

```
import numpy as np
from mayavi import mlab

image = np.random.random((100,100))

im = mlab.imshow(image,
extent = [0, 50, 0, 0, 0, 50],
interpolate=False, opacity=.3)
mlab.axes(im)
```

On the other hand, specifying the extent within the xy-plane works as expected.

```
im = mlab.imshow(image,
extent = [0, 50, 0, 50, 0, 0],
interpolate=False, opacity=.3)
```

The image positioning above could be accomplished with something like what follows, but that's very involved and requires too much maths for easy application. Also, it has to be done afterwards as `imshow` lacks these kwargs.

```
im.actor.orientation = [90, 0, 0]
im.actor.position = [25, 0, 25]
im.actor.scale = [.5, .5, .5]
```