core: Java7 Engine build

Since Java 6 is reaching its EOL, it is clear that we should start
phasing it out, and move to Java 7 where possible.

Note that the current GWT version we're using does NOT support Java 7,
so the frontend code and the code it depends on remained in Java 6.

This patch introduces the following:
1. Upgrading the maven-compiler-plugin setting to 1.7.
2. Upgrading animal-sniffer to 1.9, and using the official artifact from
   codehaus, in order to support Java 7 JDK validations.
3. Added a whitelist of com.sun classes used by existing code that
   would be picked up by the newly updated version of animal-sniffer,
   which is slightly stricter than the previous one.
   Note that these are unsupported internal classes of the JDK which
   should probably not be used externally anyway, and it would be a
   pretty good idea to remove these usages in future patches.

Change-Id: I9ea655ab589b9907184b033ea08cc98d7ee57e63
Signed-off-by: Allon Mureinik <amureini@redhat.com>
Signed-off-by: Alissa Bonas <abonas@redhat.com>