Failing high level test that uses the serialize example document
Adds single_item_arrays option to RelationMap

When this value is set to true, then RelationMap#to_hash will render
relations with only one member as an array instead of rendering that
member directly.
Adds #add_embedded_resource to Halibut::Core::Resource

`#add_embedded_resource` is used to populate embedded arrays. A
deprecation warning has been added to `#embed_resource` in the event
that it is called to populate an array.
Updates README to show preferred API
Requires all items to respond to #to_hash

Since the `#to_hash` method on `RelationMap` calls `#to_hash` on any
items that have been passed into `#add`, this has add raise an
exception if the item passed to `#add` does not meet that requirement.
Updates docs
Merge pull request #10 from mscottford/topic/single-item-embedded-arrays

Support generation of single item embedded arrays