include Android SDK version and build host OS in jar metadata
add make-release-build to automate deterministic build process
put all relevant settings into a single properties file

./make-release-build uses ant.properties to set up the signing key
compare-to-official-release: script to compare release jars

This allows anyone to test the reproducibility of the build.
remove jar signature from release, it messes up deterministic builds

No one really checks jar signatures anyway, there is a better OpenPGP .sig,
and the jar signature makes it harder to handle the determinstic build
process.
make gpg signing optional for build success