Adding specific permissions for viewers for DNEnvironment.

Allows setting of a viewer group or specific viewer users for a
deployment environment. This is useful for locking off certain
environments from being viewable (e.g. only certain users can see some
hidden environments)

If no viewers have been set for the environment, it falls back to
project view permissions.
Merge pull request #36 from halkyon/environment_view

Adding specific permissions for viewers for DNEnvironment.