Add another method based on BTLE and contact tracing

We can also exploit the contact tracing being added to iOS and Android to answer the question "how many people are around?". Cell phones are transmitting ephemeral IDs with BTLE and we can sniff that with tshark. I have described the method here:

https://xakcop.com/post/how-many-people/

If you think this fits into the project, I can start working on a PR :)