Add bthpan.rb to unstable

Closes #3651 by moving this module to unstable.

I asked @KoreLogicSecurity if he would be so kind as to test the
changes, and haven't heard back from him in 27 days. Assuming this is
abandoned.

If you'd like to reopen #3651, just say so with your results. Otherwise,
someone else can pick up this work and carry on.