DOC: point to newer function for rolling axes

We were pointing to the old deprecated function.
RF: add time and slice axes arguments to screens

The ``screens`` utility assumed time last and slice second-to-last. Get
screens to look for 't' axis (will nearly always be last, but often
set), or accept suggestion from arguments; ditto for slice axis.
RF: make default slice axis adapt to time axis

Add tests for slice axis and time axis specified.

This is minor API breakage, because previous default behavior was:

    time_axis=-2, slice_axis default -> -2
    time_axis=0, slice_axis_default -> -2

Now:

    time_axis=-2, slice_axis default -> -1
    time_axis=0, slice_axis_default -> -1
RF: add time_axis, slice_axis to nipy_diagnose

Allow nipy_diagnose to work on arbitrary slice axis
Merge pull request #268 from matthew-brett/screens-refactor

MRG: Screens refactor

I found a dataset that had the slice dimension first (the Haxby 2001 datset, ds105 on OpenFMRI.org). To do slice timing we need to be able to specify the slice dimension. These changes allow you to specify the slice dimension to the utilities for time series differences (which depend on slice).