Add the missing 'delete' command to the HashField field

The `delete` command was not in the allowed ones for the `HashField`, so it wasn't possible to use it to delete this kind of field, or to delete an object having ones (deleting an object deletes each of its fields one by one)

I had a test to check that the delete method is ok (this new test failed before my patch), and also add a test for the `delete` command for all fields.

I added a CHANGELOG file ;)

PS: don't forget to create tag for each new version, and to upload to pypi !