Add action support for Siren.
Merge pull request #4 from SebastianEdwards/master

This is good, thanks! In the future if we introduce other media types with actions it might be a good idea to abstract actions and action fields to a module or superclass.
Add failing specs for nil-entity serialization
Don't try to serialize nil with an entity serializer

If the entity (such as a reference to another object) is nil,
the `entity` method will still try to serialize nil using the
passed in block or serializer class, which will often raise a
`NoMethodError` on `nil`.

Here, we don't try to serialize unless the item actually exists
to serialize it, otherwise we return nil.
Merge branch 'handle-nil-entity' of github.com:shekibobo/oat into shekibobo-handle-nil-entity

Conflicts:
	spec/fixtures.rb
Add Serializer#map_properties to DRY property definitions

Closes #5.
Add example usage of #map_properties to README
Add more indepth description of defining property schemas
Merge pull request #6 from shekibobo/feature/map-properties

Nice. Merging. Thanks again!