Support easy access to different project configurations

[einarf]

We should support easy access to different project configurations. This way we can configure up a subset of the project when needed. When projects grow large this will be handy.

Currently the project is described in settings like this:

PROJECT = "myproject.project.Project"

Users should also be able to enter multiple projects with an alias:

PROJECTS = {
    "default": "myproject.project.Project",
    "testing": "myproject.project.ProjectTesting",
    "stuff": "myproject.project.ProjectStuff",
}

What project config config should be used can be overriden in the run command.

./manage.py run --project testing