Add support for graph spectral properties
Add support for relevant spectral properties based on the laplacian matrix:
[[Equation([l_{i,j}:=\left{\begin{matrix} \deg(v_i) & \mbox{if}\ i = j \ -1 & \mbox{if}\ i \neq j\ \mbox{and}\ v_i\ \mbox{adjacent}\ v_j \ 0 & \mbox{otherwise} \end{matrix} \right.])]]
See: http://en.wikipedia.org/wiki/Laplacian_matrix, http://arxiv.org/abs/0706.3069, http://arxiv.org/abs/0706.1176, http://arxiv.org/abs/0708.1862