Fix Broken Scaladoc Links (Resolve Linking Warnings to Third-Party Libraries)

[michaelahlers]

Warnings like these persist:

model contains 11 documentable templates
[warn] /Users/michaelahlers/Projects/michaelahlers-playful/src/main/scala/ahlers/michael/playful/json/JsValueOps.scala:9: Could not find any member to link for "play.api.libs.json.JsObject".
[warn]   /**
[warn]   ^
[warn] /Users/michaelahlers/Projects/michaelahlers-playful/src/main/scala/ahlers/michael/playful/iteratee/EnumerateeFactoryOps.scala:7: Could not find any member to link for "play.api.libs.iteratee.Enumeratee".
[warn] /**
[warn] ^
[warn] two warnings found
[info] Main Scala API documentation successful.

See also Problems with Scaladoc Linking to Classes in Comment Blocks on Stack Overflow.

[michaelahlers]

Consider using sbt-api-mappings (if and when ThoughtWorksInc/sbt-api-mappings#4 is merged and published).