Predicate operators

[torie]

This add support for using the $in and $nin operators in the query package against array fields as described in #187.

It also adds tests for nested fields and the utils.getField function.

[smyrman]

LGTM

[smyrman]

@rs, this allows the mem backend to work more equal to how MongoDB works for $in and $nin queries, making it easier to write unit-tests when using that backend. I don't have any experience with the other rest-layer backends, such as e.g. elastic-search https://github.com/rs/rest-layer-es/blob/c9cca01e407ccc54e6f84f228ac0c45700c5cf96/query.go#L81.