Join threads before return from main()
Rewrite socket interface to use a read callback

This is a preparation for issue #9.
Implement TLS for the API server

Use the Botan library to provide Transport Layer Security.
d3b now requires a X509 certificate file "server.crt" and
a private key file "server.key" in PKCS8 format.

Removes the dependency on OpenSSL.
Adds a depdendency on Botan 1.11.
Closes #9.