ENH: improve dipole plotting with MRI

see fieldtrip ft_plot_dipole.m and ft_plot_slice.m

see their tuto at : http://www.fieldtriptoolbox.org/tutorial/natmeg/dipolefitting

EDIT:

Enhancements to plot_dipole_mri_orthoview (#3966):

• Fix bound calculation in head coordinates
• We should project the orientation vector onto each of the three 2D planes that we show in matplotlib mode
• Allow backend='mayavi' to use VolumeSlicer (http://docs.enthought.com/mayavi/mayavi/mlab_case_studies.html)
• Add head mesh in mayavi version

[larsoner]

• Brain.add_mri to add the slicer
• Brain.add_dipoles to add a Dipole instance

[agramfort]

let’s make it work then we discuss API