DOC: fix data package pages for doctests

Make doctests pass by disabling some tests, fixing others, and making a
nipy test data directory framework on the fly to test against.

Some typos and bad formatting.  Reflow of paragraphs with 78 char width.
MAINT: set all .rst file as text files

Have some zero bytes when showing header printouts; tell git these are
still text files; enable diff because git still doesn't accept these are
text files, so at least show diffs on them.

Need to specify sub-components of text-ness for some reason.
DOC: move nibabel images page, fix doctests

Move nibabel image description page into main documentation.

Link downloads directory into docs directory to make doctests work.

Fix up doctests to be a little more robust with some doctest directives.

Put nibabel images page into top level of manual.
MAINT: rm reference docs for doctests; html-only

Remove the reference docs for the doctests, because they pick up lots of
failed doctests that are anyway tested in the test suite.

Add html-only target.
DOC: reword Changelog

Rewrite change log to be a little easier to read.
DOC: add images and memory page

Expand on various APIs for managing caching and memory.
DOC: rewrite docstring for get_data

Use "array image" instead of "in-memory image".  Rewrite a little with
the experience of writing the main documentation.
TST: build html and run doc doctests on travis

Try building html and running documentation doctests on travis, as a
separate entry in the build matrix.
DOC: add document on nifti format

Detailed description of stuff specific to NIfTI format.
DOC: add tutorials section

Just a couple of tutorials, but why not link...
DOC: add image orientation page

Description of image voxel orientation utilities.
RF: always close figures before plot directive

If we don't close figures, the plot directive can easily pick up figures
from previous plot directives.
DOC: add neuro / radio doc to tutorials

Fix up neuro / radio doc, I found it a bit confusing.
DOC: page on adding / using test data

Document guidelines for adding test data to main repository and as a
submodule.
DOC: document adding an image format

Notes on how you might go about adding a new image format to nibabel.
Yes, it's a little complicated.