DOC: small rewrite of gettingstarted

Refer to coordinate systems doc.  Minor rewrites.
DOC: fix Bertrand's name; slight rewrite

Slight rewrite of index to my taste.
DOC: clarify role of pip on Linux

Maybe only use pip if your package manager isn't going to do the job.
DOC: remove data package page from manual

Nibabel doesn't need the data packages, and the instructions are hard to
understand, so drop from the manual pages (but leave in the docs in case
anyone is pointing to them).
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.
RF: install sphinx in tests block

Don't install sphinx in main depends step, because sphinx is all Python,
and pip should be fine for that.