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
fast euler cycle algorithm implemented
documentation updated
compatibility with python3
Git: ignore .class files generated by Jython.
Avoid importing * for relative imports.

This is to prevent Jython from giving the following error:
'import *' not allowed with 'from .'
Don't import Graph Atlas generator if the interpreter is Jython.

The graph_atlas_g function is larger than the maximum method size
limit in Java (64K). This is just a workaround to get networkx to work
with Jython, and obviously it would be preferable to take all of the
Graph Atlas data out of the body of the function, and save it to a
separate data file.