Generating YAML used by main Software Carpentry website.

1.  Added `slug` field to `Person` (will usually be lowercase lastname.firstname) for backward compatibility.
    Only instructors need to have slugs - these are used in filenames (e.g., lastname.firstname.json is the JSON for the instructors' badge).

2.  Updated `migrater.py` and `db.sql` to reflect this change.

3.  Added links from `index.html` to displays of exported data.

4.  Added page to display exported data.

5.  Added view and helper functions to generate exported data.

6.  Modified `urls.py` to glue them all together.

To do:

1.  Pretty-print the YAML (it ought to be indented nicely).

2.  Aggregate airports so that we can count the number of instructors at each.

3.  Only select airports currently matching instructors.
Only export airports that have instructors
Fixing bugs in YAML export.

1.  Counting people at airports.
2.  Converting airport names back to plain old ASCII for display.
Merge pull request #83 from gvwilson/generate-yaml

Generating YAML used by main Software Carpentry website.