improve form type guesser

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #36
| License       | MIT
| Doc PR        | -

This makes the type guesser handle a couple more association types. Previously, anything except referenceOne/Many lead to a notice on `targetDocument` key not being defined and if php did not stop on notices things would fail later on as the 'class' option of the form is required.

Now we also handle referrers properly.