nuxeoctl should check if Java is installed

The Shell script currently check if the Java version is correct, not if it is installed.

When it is not, that currently generates the following message:

ERROR: Nuxeo requires Java 1.7+ (detected )

The issue is on OS X where the system immediately suggests an automatic install of Java (since it's not installed by default anymore on recent OS versions) and installs... a Java 6 !
So the user will then get the error another message:

ERROR: Nuxeo requires Java 1.7+ (detected 1.6.***)

The script should check if Java is installed before actually calling the command.
If not installed, it should then display install instructions dedicated to the detected OS.