Backported java.util.Objects and java.nio.charset.StandardCharsets

Objects was stripped from everything that requires JDK7 or JDK8.  That
way, tbackport-jdk8 should be usable in a JDK6 environment as well.
Use backported Objects and StandardCharsets

This should allow using this in JDK6 as well.
Set JDK source and target to 1.6