disabled removal of :private metadata in release build
restored support for previous sdk versions without build-tools

Signed-off-by: Alexander Yakushev <alex@bytopia.org>
added options to specify keypass and storepass for release build
changed apk signing to use explicitly specified passwords over default password in development builds
added dependencies for aot compilation of plugin;
resolved a bunch of reflection warnings;
added obfuscated dex command that runs proguard and dexes its output
added support for native library packaging
fixed inference of application name for run command
added auxiliary dex options
added descriptions of supported options to sample project.clj