Merge pull request #471 from ndawe/master

[MRG] plot_contour_matrix: add weights argument
Merge pull request #472 from ndawe/master

[MRG] root2matplotlib.contour: add zoom argument
Merge pull request #473 from ndawe/master

[MRG] fix get_limits for log-scale axes
Merge pull request #474 from ndawe/master

[MRG] update docs