Add directed laplacian matrix.

Code based on
https://bitbucket.org/bedwards/networkx-community/src/370bd69fc02f/networkx/algorithms/community/

I refactored Ben Edwards code to one function. This matchs the format of the
other functions in the file and allows to import numpy inside the function.
Fix test_laplacian.py name.
Add preliminary test.
Fix error in pagerank matrix.
Check alpha is between 0 and 1
Add test for directed laplacian.
Add tests for random and lazy walk.
Remove print statement.
Use require and not_implemented_for decorators.
Fix conflicts.