Fix normalized laplacian when the graph contains selfloops.

When G contains selfloops, the normalized laplacian is defined as NL = D^{-1/2}
(D - A) D^{-1/2}, with A the adjencency matrix and D a diagonal matrix with the
sum of the rows of A in the diagonal. The current NetworkX implementation use D
with the degree of the nodes in the diagonal, which count selfloops twice (this
is the right convension in general, but not the correct one for computing the
normalized laplacian).
Remove from __future__ import.
Add test and fix normalized laplacian for multigraphs.
Merge pull request #738 from aweinstein/fix_norm_laplacian

Fix normalized laplacian when the graph contains selfloops.