Add description field to network form.
Change spec to new monkey-patching-free syntax.
Connect shows to guests with through association.
Refactor networks controller.

- use local instead of instance variable to foster good delegation habits
- explicit (if un-DRY) render calls for each action
- permit a :description key to be assigned in params
- display descriptions in :index an :show views
- use a resource-based form_for instead of an action-based one
- display error messages in the form when required fields are missing