Merge pull request #407 from matthew-brett/refactor-gifti-init-and-docstrings

MRG: rewrite gifti docstrings; refactor init

Responding to comments on Ben's previous PR #403.

Expand and fix docstrings and gifti attribute descriptions from gifti
spec.

Rewrite `GiftiDataArray` init to allow full initialization of object, that
can be used from `GiftiDataArray.from_array`.  Deprecate `GiftiDataArray.from_array` as redundant.

Add tests.

Fix up the tests - there were some duplicate test names so some tests were not being run.

Remove unused `label` attribute of `GiftiLabel` object.

Author: matthew-brett