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
Whitespace.
Add is_arborescence(), is_branching().
Generalize is_tree(), is_forest() to handle directed graphs.
Add unit test for #1144.
Added tests for Jaccard coefficient
Implemented Jaccard coefficient link prediction algorithm
Added tests for Adamic-Adar index
Implemented Adamic-Adar link prediction algorithm
Added tests for preferential attachment
Implemented preferential attachment link prediction algorithm
Updated __all__
Added detailed documentation
Refined documentations
Some refactors
Reduced output precision
Added references to Sphinx source file