make and verify jar signatures in pure python, entirely remove jarsigner support

Between the various flavors of Java (Oracle, OpenJDK, etc) and the many GNU/Linux distros (let alone OSX and Windows), there is a wide variety of ways that jarsigner can be installed. Having the jar signatures made and verified using python code would eliminate that issue. Here's one example from a user on Arch: https://f-droid.org/forums/topic/binary-repo-whatsapp-error/

Here is some example code:

• https://github.com/google/keyczar
• http://penguindreams.org/blog/signature-verification-between-java-and-python/
• https://adblockplus.org/blog/signing-firefox-extensions-with-python-and-m2crypto

As a security failsafe, we can make fdroidserver also run jarsigner if it finds it.