Major changes: Library format, -R support, etc

[LuKks]

Some of the changes are:

• Can be imported as a library just like HyperDHT, Hyperswarm, etcetera
• There were 4 classes related to transfering files, now it's a single unified class
• There were 2 classes for tunnels and lacking -R support, now it's a unified class that supports both -L and -R
• Appropriate protocol messages
• Documentation is a bit clearer like less obvious texts
• Single binary with subcommands e.g. login (based on the original feedback)
• Changed the default key filename from peer to id
• Server has a different default key filename that is id_server
• Custom DHT nodes by using --bootstrap for testing
• One time invites (based on the original feedback)

Some parts of the internals that are not great but compensated with more tests:

• The tunnel.local() and tunnel.remote() methods are somewhat messy
• Strange pattern in the copy.upload() and copy.download() about re-creating an instance of self
• Can't cancel an upload or download for now
• Slightly confusing the Tunnel allow but works

It should be easy to make it compatible with Bare

Maybe the protocols (Shell, Copy, and Tunnel) should be on separated repositories in some modular way so it's simpler to reason about

Will be adding a few more tests later