fix scaladoc references

link to nested type needs to be explicit
Merge pull request #29 from dwhjames/fix/scala-doc

fix scaladoc references
Merge pull request #31 from mrvisser/topic/series-tests

Fix Series.hasValues, add test coverage
attempt at stabilizing the Travis-CI builds

 - https://issues.scala-lang.org/browse/SI-8772
 - https://github.com/sbt/sbt/issues/1439#issuecomment-51860493
Add test for Series.++
Remove unneeded s
Add tests for Series.closestKeyTo