Selfloops counted twice

Fix bug where Graph selfloops get counted twice when converting to scipy sparse matrix.
normalized laplacian scaling

No need to rescale diagonal of adjacency matrix (self loops count 1 already)
Document how to scale matrix

Add note on how to scale numpy and scipy sparse diagonals by factor of 2
Documentation for diagonal entries
Use scipy import fixture for tests
Remove duplicate XML header in GEXF/GraphML output
Merge pull request #1172 from ysitu/gexf

Remove duplicate XML header in GEXF/GraphML output
DOC: semiconnectedness
Merge pull request #1175 from argriffing/doc-semiconnectedness

DOC: semiconnectedness
Merge pull request #1078 from hagberg/diagonal-doubled

Selfloops counted twice