Add JBrowse auto-configuration.

This change set adds the ability to generate sets of JBrowse configuration files meaning that users don't have to. It means that users can simple edit their `jbrowse_conf.json` file like so:

```: json
{
  "datasets": {
    "flymine": "?data=http://www.flymine.org/query/service/jbrowse/config/7227",
    "name": "FlyMine"
  },
  ...
}
```

And all the appropriate configuration files will be generated to make JBrowse usable with this dataset.

API endpoint documentation is provided and viewable in the iodocs API explorer.

Outstanding issues:
 * Could do with more javadoc, but the classes are pretty small and mostly obvious.
 * No tests are provided for this functionality. These should be integration tests anyway, sine we are dealing with an external API contract. On the other hand, this is a very small thin controller over well tested API code.