Register multiple Jarfiles to merge and lock as a unit

This allows me to define a library that depends on other libraries that also use lock_jar without needing clients to use any tools to prebuild a Jarfile.lock or manually combine the dependent libraries' Jarfiles beforehand. 

Instead, each Jarfile can be registered using `LockJar.register_jarfile(jarfile_path)`. Then all of the registered Jarfiles can be merged by calling `LockJar.lock_registered_jarfiles`. Finally, `LockJar.load` can be called as usual.