Update spectrum.py to use scipy.linalg
Split matrix conversion functions into new module
Simplify import statements
More efficient to_scipy_sparse_matrix

Doesn't convert to LIL format
Convert laplacian and normalized_laplacian to sparse
Switch adjacency_matrix to sparse
Update incidence_matrix to sparse
Merge branch 'master' into use-sparse-matrix

Resolve conflicts in eigenector.py and katz.py due to switch to sparse matrix in adj_matrix.

Conflicts:
	networkx/algorithms/centrality/eigenvector.py
	networkx/algorithms/centrality/katz.py
Use todense() to match adj_matrix
Use sparse matrix
Directed laplacian to sparse

Mostly done conversion of directed matrix laplacian to sparse matrix format.
The "pagerank" walk is still using dense matrices.
Use spdiags instead of diags

Diags is a new scipy function?  Not found in the version on travis for testing.
Skip tests if no SciPy
More skip tests with SciPy
Merge pull request #1021 from hagberg/use-sparse-matrix

Use sparse matrix