Add the `isReadOnly` option for creating and updating checkbooks.

+ Add the `isReadOnly` param to `update` and `create` in
`CheckbooksResource`
+ Update the various tests

fixes #8
Add `on_behalf_of` to `FormSubmission`.

+ Add the `OrgSnippet` (e.g. `portal_snippet`)
+ Add the `onBehalfOf` field

closes #10