Use Keyword views to R/W ref attributes

Make the `ToDRef` type class more flexible: allow types viewable as `Keyword` to be lifted into a `DRef`.
and similarly replace `Attribute2EntityReaderCast[DRef, Card, Keyword]` with `Attribute2EntityReaderCast[DRef, Card, K]` with implicit `Keyword => K`.
Merge pull request #77 from pellucidanalytics/topic/rw-kw-implicits

Use Keyword views to R/W ref attributes