Update sbt-extras cookbook

Most Important Changes:
* almost ready to integrate with official paulp/sbt-extras
* sbt 0.13.0 is now default launcher
* better pre-installation of sbt and scala boot libraries
* verbose mode enabled to better show which sbt and scala versions are used during the build
* custom JVM options simplified, since sbt-extras now include some of our tweaks.
Merge pull request #220 from gildegoma/update-sbt

Major updates in sbt-extras cookbook