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
Fix sample_condition so events with onset=0 are not ignored
Started to refactor temporal subsampling to allow negative and numm onsets
BF: Fixed the tests to take into account min_onset + made min_onset relative to frmateimes[0]
ENH: Removed the constraint on frametimes and the corresponding test
ENH: Took this opportunity to clean test_hemodynamic_models
ENH: idem with test_dmtx
ENH: took this opportunity to remove an unsued variable n_event in Paradigm class
Added warnings so that events are not silently ignored
BF: added two tests... and corrected some bugs in the warnings
ENH: expose the min_onset parameter
ENH: addressed the PR comments, pep8
BF+TST: make sure labels stay in order for exclude

Previously passing ``exclude`` to the ``parcels`` generator would make
the order of labels undefined, because we use ``set(labels)``.  Keep the
order of labels; test that we do.
NF: add MNI basal ganglia definitions for example

Process MNI template atlas a little to get basal ganglia definitions.
Make README with MNI data license.
NF: add example for parcels generator

Small example to show extracting mask for unique ROI values in an image.
BF+TST: check passing of more than one exclude

We claimed to accept iterables, but in fact numpy arrays failed as input
because of the ``if exclude:`` line.
RF: faster check for excluded label
Merge pull request #275 from jonathan-taylor/ds105_analysis

MRG: ds105 fmri example with single runs being fit

* Modified the FIAC example to start trying to analyze https://openfmri.org/dataset/ds000105
* Added a block_design function, and options to output individual terms for an experimental factor.
Merge pull request #269 from matthew-brett/test-doc-parcels

MRG: test exclude keyword, document parcels

* Rewrite docstring for parcels. 
* Change exclude argument to be tuple to avoid mutable default argument.
* Using 'exclude' not previously tested - test
* Add example for using parcel generator
* Example revealed bug in label sorting in `exclude` case; fix + test.