I made a quick C# utility to take a plugin/behavior directory and make a .c2addon file. It's a command-line tool that works with drag & drop (including multiple directories)
i.e. if your plugin is in
it makes c:\myplugin.c2addon
The tool verifies that info.xml and the "files" subdirectory exists before attempting to do the zip.
This requires .NET 4.5 assemblies, so you need Windows 8, Visual Studio 2012, or some other way to get .NET 4.5
I have included the source, not a binary, since Visual Studio Express 2012 for desktop is free, and no one should run random .exe files from the net. :)