Add support for ObjectId

[stIncMale]

HIBERNATE-23

[stIncMale]

This way it is easier to express what exactly we are expecting than with a string.

[NathanQingyangXu]

Looks great overall. Only some minor comments are left.

[NathanQingyangXu]

Just found an edge case wherein explicit @JavaType annotation is used, the getById() testing case would fail:

@Entity
    @Table(name = "items")
    static class Item {
        @Id
        ObjectId id;

        @JavaType(ObjectIdJavaType.class)
        ObjectId v;
    }

[stIncMale]

Regarding #70 (comment)

Just found an edge case wherein explicit @JavaType annotation is used, the getById() testing case would fail

This is because Hibernate ORM instantiates ObjectIdJavaType on its own in that case. Consequently, when it calls ObjectIdJdbcType.getBinder, the instance of ObjectIdJavaType passed as the javaType argument is not the same as ObjectIdJavaType.INSTANCE. I added the ObjectIdJavaType.equals/hashCode overrides to take this into account. However, applications can't explicitly use ObjectIdJavaType, ObjectIdJdbcType, because they are both internal. Are you aware of any reason to make them part of the API now?

Done in 60c7345.