Add resource_class to Her::Model::ORM

Different APIs may have different conventions for searching for relations.  For
example, an API may not use query params for `#where` like searches. In these
cases, it might be useful to have a model use a specialized `Relation` class
that contains these rules.  This commit adds two methods to `Her::Model::ORM`:

- `.use_relation(klass)` specifies the relation class to be used
- `.relation_class` gets the used relation class

Internally, the ORM module uses the defined relation (defaulting to
`Her::Model::Relation`) for setting up queries.