Add support for signed XML plist files

[joachimmetz]

Add support for XML Plist files with signature:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
...
</dict>
</plist>
----BEGIN RSA SIGNATURE----
<binary data containing signature>