Add an Adopters section

Make it easier for people to see the adopters of the technology and point them to the Powered By wiki for the full list. Also make it easier for them to add themselves if they wish.
Remove Java Commons DateUtils dependency from Scalding
Remove unused import
Removed commons-lang dependency from date module
enforce version 0.6.2 of sbt-git to work around https://github.com/sbt/sbt-git/issues/19