now honoring spray.httpx.compact-json-printing setting

The [configuration documentation](https://github.com/spray/spray/wiki/Configuration) mentions that the boolean _compact-json-printing_ can be set to true so that the default json marshaler will use _CompactPrinter_ rather than the default _PrettyPrinter_. This however doesn't seem to work in the latest versions.
Looking at the current version of [SprayJsonSupport](https://github.com/spray/spray/blob/master/spray-httpx/src/main/scala/spray/httpx/SprayJsonSupport.scala) we see that the default printer is set directly to PrettyPrinter. Searching through the code, it seems that the compact-json-printing settings is no longer used. The latest time when this worked seems to be in [version 0.8](http://sassunt.github.io/sxr/spray_0.8.0/spray-base/cc/spray/typeconversion/SprayJsonSupport.scala.html)
This patch loads and uses *spray.httpx.compact-json-printing* to select the default printer. The *spray.httpx* sub-config was chosen to maintain the latest config style that seems to use the spray module name, not using any more settings directly from the *spray* config.