useReferences semantics

[fizruk]

Now that #19 is merged we can change useReferences semantics or even get rid of the option altogether.

Currently useReferences only affects schemas reachable from GToSchema, so e.g. for this type Object is not reachable:

data Objects = Objects [Object]

With declareNamedSchema we can now inline every schema.
Moreover since we have access to all used schemas and we can choose which one to leave and which one to inline after calling declareNamedSchema.

In a bit more general setting, we might implement this function:

-- | Inline schemas with given names and remove them from definitions.
inlineSchemas :: [Text] -> Definitions -> Definitions

This can be useful for tweaking final Swagger spec.