Add port of random fields tutorial

Partial port, with remaining bugs, from
http://imaging.mrc-cbu.cam.ac.uk/imaging/PrinciplesRandomFields and
http://imaging.mrc-cbu.cam.ac.uk/scripts/randomtalk.m
Fixed simulation and some indexing

I had missed a scaling factor for the smoothed images from the MATLAB
script.

Discovered some incorrect indexing too.  Script now correct, but needs
more text.
Merge branch 'random-fielding' of https://github.com/matthew-brett/bayfmri

* 'random-fielding' of https://github.com/matthew-brett/bayfmri:
  Fixed simulation and some indexing
  Add port of random fields tutorial
Merge branch 'master' of github.com:jbpoline/bayfmri

* 'master' of github.com:jbpoline/bayfmri:
  More motion QA plots. Added pre-computed motion displacements.