Add title addribute to siren action's fields

This is a very small PR which allows adding the `title` attribute to `fields under actions` in the Siren adapter.
As seen in the [Siren docs](https://github.com/kevinswiber/siren#fields-1)

Spec included

P.S
Took the liberty of converting the siren spec to the new hash syntax (on the 2nd commit), it looks much cleaner.