Compile QFJ 1.6.x against JDK7 and QFJ 1.7.x against JDK8

We should compile at least against JDK7 for various reasons:

• JDK6 is quite old now and end-of-life (actually, even JDK7 is)
• some of the SSL tests do not work with JDK6 since it does not support all ciphers
• MINA 2.0.15 is compiled against JDK7
• some smaller improvements in the code could be made (multi-catch, diamond operator, ...)

Since there only is a 1.7.0-SNAPSHOT release now (i.e. no released version), we can simply compile against JDK8 there.