Test the Java version in the startup script

When trying to test the 8.1M2 in its standalone version I've been put in a strange state where :

• the Java info is telling me that i'm on java 8.x
• the online browser test is telling also java 8.x
• the %JAVA_PATH% env variable is still at 7.x

Reading the error given was not as clear as telling me that I needed to check my env var from my OS :

Caused by: java.lang.UnsupportedClassVersionError: org/xwiki/tools/jetty/listener/NotifyListener : Unsupported major.minor version 52.0

Proposal : Test the Java version in the startup script and if it's the wrong one, warn the user of what to do instead of sending the error.