DOC: add new formats to README / long description

Add notes of nifti2 / minc2 support.
Also add one-liner first line description for API docs.
BF: add optpkg fix for odd import error

This fix comes from dipy; it was in order to deal with cvxopt, but it's
probably sensible in general.
DOC: fix footnote ref in data package discussion
DOC: fix misc docstring formatting

Add /fix one-line descriptions of modules.

Fix bad formatting in lists in docstrings.

Fix misspelled docstring section headings.
DOC: switch to mathjax for math rendering

Makes the doc build faster; page load should also be quick because we
were using a lot of generated math PNG images.
RF: remove copy of sphinx autosummary

sphinx has this in version 0.6 (but it seems it was a little broken).
Version 1.0 released in 2010, so we should be safe now.
NF: add apigen stuff from dipy / skimage

Edited to make a little more general, and customize for nibabel.

Thanks Stefan vdW for keeping this working well.
RF: use apigen machinery for docs

Use dipy / sklearn machinery to generate the API docs.

Fixup `api.rst` to include some more modules, and add full API
reference.