Create data_type model
Migration created

Ran:

```
python manage.py schemamigrations datasets --initial
```
using --initial as it was the first model of datasets
subsequently use --auto
(after that to apply the migration:)
```
python manage.py migrate datasets
```
Add tests for DataType class.

Note that these require django-nose instead of the stock django test
runner, due to the use of generator tests (see nose docs)
Register data type with admin app
Merge pull request #11 from alphagov/data-types

Data types