introduce writeFact and deprecate writeAttr

this replaces
(__ \ “foo”).write(writeAttr[T](attr))
with
writeFact[T](__ \ “foo”, attr)
handle binary data from Datomic

- write a byte array as a base64 encoded json string
- play 2.2.x includes a rather ancient version of commons-codec, so the `Base64.encodeBase64String` convenience method is not available.
Merge pull request #5 from pellucidanalytics/topic/write-json-fact

introduce writeFact and deprecate writeAttr
Merge pull request #6 from pellucidanalytics/topic/binary

handle binary data from Datomic
Merge branch 'master' into topic/implicit-format-keyword

Conflicts:
	src/main/scala/play/modules/datomisca/Implicits.scala