NF: add copy of OrderedDict for Python 2.6 compat

DICOM will need OrderedDict; copy in the external module until we
require Python 2.7
Merge pull request #239 from matthew-brett/add-ordereddict

NF: add copy of OrderedDict for Python 2.6 compat

DICOM will need OrderedDict; copy in the external module until we require
Python 2.7
DOC: add tutorial on affines

Fairly in-depth tutorial about affine matrices and reference spaces.
DOC: expand and revise affine tutorial

Make explicit the components of the affine transform using the EPI
affine example.
BF: update matplotlib plot_directive with fixes

Add fixes from refactor of plot_directive
BF: fix coordinate_systems build with plot options

Missing plot_directive options in conf.py