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.