Allow to configure a jongo mapper factory for customized jongo mapper.

This adds the optional "playjongo.mapperfactory" configuration which accepts
the name of a class implementing uk.co.panaxiom.playjongo.JongoMapperFactory.
The factory class must also provide a default constructor.
Add JongoScalaMapperFactory for jongo/jackson scala support.

Uses need to add
  `"com.fasterxml.jackson.module" %% "jackson-module-scala" % "2.2.1"`
to their build appDependencies and
  `playjongo.mapperfactory="uk.co.panaxiom.playjongo.JongoScalaMapperFactory"`
to application.conf.
Merge