Modify SBT to have api mapping to Scala library for scaladoc

[ashawley]

This is the solution that I ran across here:

http://stackoverflow.com/questions/18747265/sbt-scaladoc-configuration-for-the-standard-library

[SethTisue]

LGTM. I see in the Travis log there are fewer warnings during Scaladoc generation now.

a few still remain for other libraries:

[warn] /home/travis/build/scala/scala-xml/src/main/scala/scala/xml/parsing/FactoryAdapter.scala:165: Could not find any member to link for "org.xml.sax.SAXException".
[warn]   /**
[warn]   ^
[warn] /home/travis/build/scala/scala-xml/src/main/scala/scala/xml/include/XIncludeException.scala:13: Could not find any member to link for "java.lang.Throwable#getMessage".
[warn] /**
[warn] ^
[warn] /home/travis/build/scala/scala-xml/src/main/scala/scala/xml/include/sax/EncodingHeuristics.scala:39: Could not find any member to link for "IOException".
[warn]   /**
[warn]   ^
[warn] three warnings found

[ashawley]

Yes, a few warnings related to the Java standard library remain. There might be a way to handle those references, but I couldn't find one for SBT. We could take it up later in another PR.

[SethTisue]

We could take it up later in another PR

indeed, no reason not to go ahead and merge this one

[SethTisue]

needs rebase now. LGTM otherwise and I would suggest we merge in a few days, after the rebase, if there are no actual objections

[ashawley]

Rebased, travis build succeeds, and there are still fewer scaladoc warnings.