Adding a 'new' function to create images from scratch.

`skimage.novice` now has a `new` static method that creates a new image of a given size (and optionally color) from scratch.
* Complicates the constructor of `skimage.picture` slightly.
* The `__format` field of a from-scratch image is `None` until that image is saved.
* Reopening the image after a `save` to find its format seems clumsy --- can this be fixed?