Implement "Export Signed Package" tool.
Supports the creation and loading of keystores and keys / certificates.
Only BKS keystores are supported at the moment, although the ZipSigner
library might provide an easy way to support JKS keystores. This feature
requires some more ZipSigner JAR files which in turn require the
SpongyCastle library. This makes the APK file a great deal larger, but
ProGuard should take care of that once we get around to using it...