Adding airports to pages.

1.  Adding classes needed for Django form creation.
2.  Adding URLs for forms and pages.
3.  Putting editable fields for airports and other objects into free-standing variables.
4.  Added views.
5.  Added headings to views of other "all_xxx.html" pages.
Starting to work on tasks
Merge branch 'adding-airports' into adding-tasks
Adding display of tasks - editing still fails.

1.  Added usual views for table-based display and forms-based editing.
2.  Link to edit tasks fails - Django complains that it cannot look up the task, but is being given the id.
Set pk_url_kwarg on tasks view so django knows where to look for task
id.
Get the right reverse url for a task object using kwargs.
Make the edit url for a task object a bit more friendly.
Line too long in Task's get_absolute_url method
Added tests for the new Task views
Some comments from @gvwilson
Merge pull request #55 from gvwilson/adding-tasks

Adding tasks - editing not working because of task lookup problems.