Improvements to scaladoc generation

- fix dependencies for aggregated build (should be including Datomic)
- turn on scaladoc -implicits flag
- turn on source linking in scaladoc and link to git tree on GitHub via tags, using the -sourcepath and -doc-source-url flags
- build an api mapping for doc.datomic.com/javadoc
- fix javadoc links using http://stackoverflow.com/a/25070934