RF: make verbose printing depend on env var

Verbose printing for realignment can be distracting; make it depend on
the NIPY_DEBUG_PRINT environment variable.
NF+FAIL: try to test slice timing

Trying slice timing in no-motion case; ascending alternating tests
failing for some reason.
BF: oops - mixed up space and time again

I was using the time order rather than the space order; these turn out
to be the same for ascending and descending but not interleaved, hence
the error.
Merge pull request #286 from matthew-brett/trying-time-testing

MRG: writing a test for slice timing

A test for slice timing in the no-motion case, by taking some smoothed time 
series sampled at higher resolution,and seeing if I could estimate the
generated time point corresponding to the start of the TR, by giving 
the routine the time points sampled at times corresponding to the slices.