overhaul layer to use new features of reactive, apt layer fixes #10

[marcoceppi]

The largest change here is the deprecation of charms.django for the better, charms.layer.django. The deprecation process means those importing charms.django should continue to work until I get tired of having it.

Furthermore, this deprecates django.yaml as the means to supply configuration for layer options instead. To upgrade, simple copy your django.yaml contents to layer.yaml under the following keys:

options:
  django:
    YOUR-DJANGO-YAML

with the exception of the apt-packages key which should instead be placed as such:

options:
  django:
    YOUR-DJANGO-YAML
  basic:
    packages:
      - list
      - of packages