!htp #15656 split out scala-xml support into its own subproject
+pro #15917 provide SprayJsonSupport in akka-http-spray-json project

This will serve as an example of how to provide similar glue layers for
other JSON libraries.
Merge pull request #16205 from spray/w/15656-fine-grained-dependencies

http: #15656 #15917 split out and provide glue layers for scala xml and spray-json support