Plotting fixes & enhancements

[ejolly]

We should clean up plotting and updated methods to make it easier to tweak the underlying nilearn calls. We should also fix-up plotting with Adjacency:

Some relevant issues:

• add vmin, vmax, cmap options to plotting functions #378
• Adjacency.plot_mds errors with n_components = 3 #381
• Plotting thresholded data doesn't work reliably #372
• Brain_Data iplot and plot produce different percentile thresholding #311