lib/build.xml: Also set javac source to java6

"javac: target release 6 conflicts with default source release 1.7"
error will be emitted if a java7 compiler is used and target is set to 6
but not source.
lib/build.xml: Add comment why we use java6 target/source
Merge pull request #90 from Flowdalic/fdroid

Final f-droid related modifications