Make Context.apply() work automatically with DAs.

Now `apply()` extracts the `.key` attribute when using `apply()` on a
DistArray instance.
Merge pull request #585 from enthought/bugfix/apply-extract-da-key

Make Context.apply() work automatically with DAs.
Merge branch 'master' into feature/lazy-evaluation

Conflicts:
	distarray/globalapi/context.py
	distarray/globalapi/tests/test_context.py