Downgrades project JDK targets to Java 7

Anything on the Raspberry Pi will be running Java 7 rather than 8. We don't
need anything from Java 8 at this stage anyway. If we need to, we can
always upgrade the ground station stuff later on if we need to.
Adds in auto-generated jetty-web xml file for now

Netbeans keeps adding it in automatically. It doesn't actually appear to
be needed to for the web service, and as far as I'm aware it's not used.
I've added a comment to it for now to illustrate this. I could have added
a .gitignore, but it would still be added to the file system by Netbeans,
even if not the repo. I think this is a little clearer.