initial adamic-adar similarity implementation
corrections and test
Revert "corrections and test". Pushed in wrong branch.

This reverts commit 415fdccbe93f465806ef0a03c5b9263671e47461.
Revert "initial adamic-adar similarity implementation". Pushed to wrong branch.

This reverts commit a65f8eb86b885fd61bc31b89168de7138b77ebd4.
Merge branch 'second-level-semimetric' of github.com:vasia/okapi into second-level-semimetric
seems to be working, needs tests and optimization :)