expand Writes[Keyword] instance into a Format instance

- check for types other than string, empty, and not beginning with a colon
Merge branch 'master' into topic/implicit-format-keyword

Conflicts:
	src/main/scala/play/modules/datomisca/Implicits.scala
Merge pull request #7 from pellucidanalytics/topic/implicit-format-keyword

expand Writes[Keyword] instance into a Format instance