Fix global-to-local indexing and local-to-global indexing.

See gh-issue #154
Test for gh-issue #154
Merge pull request #177 from cowlicks/global-to-local-fix

Fix global-to-local indexing (see gh-issue #154).
Merge branch 'master' into cowlicks-reduce-tests

Conflicts:
	distarray/tests/test_client.py
Move Context class to its own module, context.py.
Change imports to reflect moving of Context class.
Remove old copy of Context class form client.py module.
Change __init__.py to reflect omve of Context class.