Add a "how to install JDK" documentation

I have tried to install Jahia after having removed my JAVA_HOME parameter. I get the following message in my console:

Either JAVA_HOME does not point to correct location or the JDK installation is invalid, or JIKES couldn't be found.

In the startup script, we display this message when we detect that this parameter is not defined. We could add a line to open an html page in the web browser, so that we display some more understandable information to the users who are not used to java installation. We could also add in this html page a direct link to jdk download.

Test done for windows with jahiaInstallation.bat (added last line) :
:invalidjdk
echo Either JAVA_HOME does not point to correct location or the JDK installation is invalid, or JIKES couldn't be found.
start file://%CD%/install_jdk.html

We could also merge jahiaInstallation and jahiaServer in a single startup script, doing the following tests:
if JAVA_HOME is not defined -> we display the howto_install_jdk html page
if JAVA_HOME is defined and no ROOT folder is present in webapps/ -> we open http://localhost:8080/config (as currently done ny jahiaInstallation script)
if JAVA_HOME is defined and a ROOT folder is present in webapps/ -> we open http://localhost:8080/ (as currently done ny jahiaServer script)

We also need to document how to update this script in the "How to change Jahia application context" and "How to change Jahia appplication port" documentations (by the way, do we have such documentations?)

We should also do some cleanup, because there are a lot of files in the root folder (is it really mandory to have jahia-sync-nodes.xml at this level? )
We could also have a windows and a mac/unix folders, with start.bat and stop.bat in the first one, and start.sh and stop.sh in the second one. It would be more understandable for non geek users.