NF+TST: add mmap keyword to array_from_file

Add memory mapping keyword to array_from_file getting ready to add this
to the image loading API.
NF: add mmap keyword to ArrayProxy class

Give array proxy class ability to tune memory mapping via `mmap`
keyword-only argument.
NF: add mmap keyword for Analyze image types

Allow specifying mmap flags when loading Analyze-type images.
NF: top-level load passes on keyword arguments

Check this works for mmap keyword argument to top-level load.
RF: refactor arrayproxy tests for re-use

I want to use the arrayproxy tests for other ArrayProxy objects,
particularly the PARRECArrayProxy. Refactor to make this easier.
RF: refactor image mmap test for re-use

Refactor image arrayproxy test mixin to make it easier to deal with the
case where we can't easily make a new image from scratch, and we might
want to use some example image instead.
NF: add mmap keyword to PARREC with tests

Use refactored tests to test PARREC mmap interface.
DOC: rewrite docstrings using `infile`

I copy / pasted docstrings leaving the original argument name `infile`,
for methods / functions where this is not the argument name.  Fix.
RF+DOC: PEP8 / docstring fixes from Eric's review

Use 'A is not None' instead of 'not A is None'.

Fix incorrect docstring mentions of 'r+'.
Merge pull request #268 from matthew-brett/mmap-api

MRG: mmap keyword for loading images

Add mmap keyword to control memory mapping on image load.

API as discussed on the nipy list.
RF: change absolute to relative imports

Use relative imports for nibabel modules.