Issue 1453: MDS: fall back to SVD when the full similarity matrix is known

The SVD method can now be used to calculate the metric MDS. In order to do this a new argument ``method`` which defaults to ``smacof`` and can be set to ``svd`` was introduced in the MDS estimator.
This was done in order to fix issue #1453.
You should use [sklearn.utils.extmath.norm](http://scikit-learn.org/stable/developers/utilities.html#efficient-linear-algebra-array-operations) instead.