DOC+TST: test exclude keyword, document parcels

Using 'exclude' not previously tested.  Rewrite docstring for parcels.
Change exclude argument to be tuple to avoid mutable default argument.
ds105 fmri example with single runs being fit
redesign similarty measure object to allow different normalization strategies
update affine registration example
fixed python 3 issue
allow input transforms as arrays for resampling
some renaming for clarity
add test
avoid modifying input arg in explore method of HistogramRegistration
add gradient and hessian computation methods to HistogramRegistration object
pep8 editing
add docstrings
rename flag argument normalize==True as renormalize==False for clarity
add test for gradient and Hessian computation of registration similarity function
modifs suggested by Matthew
fix clumsy test
Fix sample_condition so events with onset=0 are not ignored
ENH: visualization deals with empty maps

Not crashing on empty maps is useful, as they pop up every once in a
while
Started to refactor temporal subsampling to allow negative and numm onsets
NEWFEATURE: get_cut_coords(...) function to heuristically compute good cross-sectional cut_coords (see demo in localize_glm_ar.py)