support keywords as a Datomic data type (resolves #40)

extend DatomicData with DKeyword (which wraps Keyword)
extend conversion implicits
fixup tests
sweep up other cases of DKeyword vs. DRef

hopefully the code base is more consistent in its use of Keywords and ident references.
Merge pull request #58 from pellucidanalytics/feature/keyword-type

Add support for Keywords as Datomic data type