Add png/jpg read image type support

[Hboni]

For now, Francis only manages nifti files, but it can be interesting to add support of more image type. I first thought about png and jpg format.

For jpg images, we can use the skimage.io.imread function. For png, I found that the imageio module can help using imageio.imread method (as described here)

[Hboni]

I was thinking about the display of this type of image, maybe the display will need some refactoring to be able to see RGB image? Or we can only use greyscale images in the first place?

[jeaunrg]

We can now visualize RGB images but we need to adapt model methods for this kind of images...