Adding some things I needed

Please don't merge yet, this is just a request for comments. Some things are broken(see my notes).

I got it working perfectly with my other code though, it looks much neater than before too!
You shouldn't repeat the call to `jack_set_process_callback()`.
It should be enough to properly set the variables `callback_wrapper` and `userdata` and then make the library call (using said variables) outside of the if-clause.