Simplify the package structure so it's easier for users (and developers).

Our current package structure requires knowledge of the source code file hierarchy in order to use nipy. Our import statements are long and cumbersome. I think a user (and developer) would like to do something like:

import neuroimaging as ni
img = ni.load(ni.anatfile)
ni.SliceViewer(img)

Instead of:

from neuroimaging.core.image import image
from neuroimaging.testing import anatfile
from neuroimaging.ui.sliceplot import SliceViewer
img = image.load(anatfile)
SliceViewer(img)

The sooner this is fixed the better so all our documentation and examples conform.

This should be discussed and some agreement reached by the team before implementation.