Includes six.py module in distarray.

Fixes all imports.  Includes six's license in docs.
Merge branch 'master' into feature/include-six

Conflicts:
	.travis.yml
	distarray/client.py
	distarray/local/denselocalarray.py
	distarray/odin.py
	distarray/tests/test_client.py
	examples/odin_demo.py
Moves six.py to distarray/externals package.

Makes clear that six is not part of distarray, and allows indirection
down the line to use an already installed six.
Merge pull request #145 from enthought/feature/include-six

Includes six.py module in distarray.