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.
This will have to rebased on top of master and replaced by a call to `check_array`, see:

http://scikit-learn.org/stable/developers/utilities.html#validation-tools