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.
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 #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.