Major updates in sbt-extras cookbook

@michaelklishin could you review this please?

Most Important Changes:

* We now need to export SBT_OPTS and JVM_OPTS environment variable, to refer to default configuration files. This step is required to be able sooner or later to integrate with official paulp/sbt-extras repository, instead of my fork.
* sbt 0.13.0 is now default launcher (note that sbt 0.13.0 is already possible to be used in travis production, due to sbt-extras flexibility).
* Better pre-installation of sbt and scala boot libraries (same libraries are no more duplicated, full preinstallation of scala boot libraries). In total it will cost about 500M of preinstalled stuff (versus 300M in current Travis CI production), but it will speed up the scala builds by reducing download time. Note that, we also could drop some versions, but I think the current list still makes sense (except 2.11.0-M5)
* custom JVM options could be simplified, since sbt-extras now include some of our tweaks.
* verbose mode enabled to better show which configuration is used during the build (it does not make the sbt/scala output more chattier, only the launch step).

```
vagrant@precise64:~/tests/spray$ sbt -Dakka.test.timefactor=1.5  test
Detected sbt version 0.12.4
Using /home/vagrant/.sbt/0.12.4 as sbt dir, -sbt-dir to override.
Using jvm options defined in file /etc/sbt/jvmopts
# Executing command line:
/usr/lib/jvm/java-7-oracle/bin/java
-Xms2048M
-Xmx2048M
-Xss6M
-XX:MaxPermSize=512M
-XX:ReservedCodeCacheSize=256M
-Dakka.test.timefactor=1.5
-jar
/home/vagrant/.sbt/launchers/0.13.0/sbt-launch.jar
test

[info] Loading project definition from /home/vagrant/tests/spray/project
```