BF: delete objects to satisfy Windows

Delete objects to prevent errors deleting temporary directory in
Windows.
DOC: reformat, add to some docstrings / comments

Reformat for 78 character length; one typo.
Add some extra comments.
BF: remove scipy decorator from mixin tests

I was using the decorator to say that that tests need scipy inside the
scaling tests mixin, instead of in the main test function for Spm99,
where this is actually true.  Hence tests were being skipped for Nifti1,
Nifti2 if scipy missing, when then need not be skipped.
RF: rename ScalingMixin to ImageScalingMixin

We already had a HeaderScalingMixin.
BF: work round horrible MSVC uint64 casting

MSVC casting floats to uint64 casts floats > 2**63 to 2*63, at least on
32-bit Windows.  Platform standard compiler.
BF: avoid relpath in test if not needed

relpath giving errors testing on Windows when code not on the
same drive as test.  Avoid using relpath unless the relpath really needs
to be ``..``, in which case test will still error, but as a way of
failing.
BF: fix travis builds

The virtualenv command was always using Python 2.7; get virtualenv to
use virtualenv python version; debug print check.
BF: bytes for PAR header content in parrec2nii

The extension contents needs to unpack to bytes, because we are writing
into a binary (header) file.
DOC+RF: more comments / rewrite of scaling checks

I found the scaling checks difficult to follow, so I added some comments
and refactored the scaling checks for the array_to_file routine.
Merge pull request #289 from matthew-brett/nasty-windows-fixes

MRG: nasty windows fixes

An absolutely horrible bug for MSVC casting from float to uint64 caused some
test failures on the buildbots.

Debugging, it did seem the code could work more robustly in a couple of cases.
Otherwise test for this horrible misfeature when present.

Some formatting changes and a fix for orderreddict testing when on Windows.

Delete temporary files to satisfy Windows' more stringent file attachments.