Remove machine prefix from namespace.

This change removes the 'machine' namespace, thus making the path for
most modules a bit shorter. For example, learn.machine.ann becomes
learn.ann. In my oppinion, this namespace was superfluous and there is
no loss in removing it.

This commit also renames manifold_learn namespace to manifold.

As a side effect, most modules had to be moved, and this commit breaks
backwards compatibility.

From: Fabian Pedregosa <fabian.pedregosa@inria.fr>

git-svn-id: https://scikit-learn.svn.sourceforge.net/svnroot/scikit-learn/trunk@398 22fbfee3-77ab-4535-9bad-27d1bd3bc7d8