Allow custom backup rules

Some apps require additional steps in order to make a working/usable backup. To mitigate this issue, custom rules will be allowed. The rules may vary vastly between apps and therefore, allowing only a specific set of rules is problematic. As a result, AM will support Java classes instead of any specific rules file or a typical shell script. Java classes will have full access to root environment (will be loaded via DexClassLoader) and have some specific packages and methods that AM will invoke. Details will be published later but the classes must be wrapped up in a jar file and its source (must be under a free license) has to be verified and have specific jar signature (release version of the jars can only be built by me) present in order to ensure that they're safe to use. The jar contents will be loaded during before, after or during the data backup/restore and can also override the data backup/restore operations. But the backed up files must be reported to AM in order to provide encryption support. The jar file will also be backed up during the backup process and will be loaded during the restore process and will be encrypted. The skip signature flag will not effect the verification of the jar file to ensure its safety.