Refactor picture.rb#default_mask

Returns the default centered image mask for a given size.
If the mask is bigger than the image, the mask is scaled down
so the largest possible part of the image is visible.

The function will always return a Hash where all values are Integer.

At this point, we can't know what Jcrop is going to do to all this,
so Jcrop will have to scale down the default_mask to whatever
it needs.
Move #default_mask to essence_picture.rb

Since ```EssencePicture``` holds the data for cropping and only
```EssencePicture``` can be cropped with a mask, I do think that
the ```#default_mask``` method should be on EssencePicture.

In the test for the ```#crop``` method, ```@picture.default_mask``` was stubbed
with values that cannot possibly arise using the current ```default_mask```
method. This commit sets them to the dimensions of the full image.