Add a function to calculate the signless Laplacian matrix

The signless Laplacian matrix is a slightly different version of the
Laplacian matrix and is used in some parts of spectral graph theory.
http://dx.doi.org/10.1016/j.laa.2007.01.009 describes it.
Merge pull request #18 from andrioni/master

Signless Laplacian matrix
Add basic dot file output
Merge pull request #19 from rsofaer/write_dot

Add basic dot file output
Write Vertex attributes
Merge pull request #20 from rsofaer/dot_vertices

Write Vertex attributes and add plot function