Improving the documentation, test coverage and API usability.

`django_node.node.run` and `django_node.npm.run` now accepts *args, rather than a single tuple.
`django_node.node.is_installed` is now `django_node.node.is_installed`
`django_node.npm.is_installed` is now `django_node.npm.is_installed`
Removed the `DJANGO_REACT['ALLOW_NPM_INSTALL']' setting.
Bumping the version to 0.2.0.