First pass for layout.jl.
Added sparse adjacency, weight and laplacian matrix functions.
Added helper functions to layout.jl.
Merge branch 'layout'
Minor update to layout.jl.
Missing 'end' in gmatrix.jl.
Fixed name of weight_matrix_sparse.
Export sparse functions.
Added tests for adj_matrix_*_sparse.
Fixed some bugs in adj_matrix_*_sparse based on tests.
Added tests for weight_matrix_*_sparse and laplacian_*_sparse.
Removed warnings to test.
Merge pull request #38 from nfoti/master

WIP: Sparse adjacency, weight, and laplacian matrices and some layout functions.