Add support for generating self-signed TLS certificates

[roblillack]

This makes sense in a production environment where the connection between the backend (Mars) and the frontend (ie. AWS Application Load Balancer) server shall be encrypted and it does not make sense to manually update certificates all the time.

[roblillack]

Implemented this together with dual-stack HTTP support.

The following config options where added:

• http.dualstack (default: false) to enable dualstack mode (serving HTTP and HTTPS).
• https.addr (default: ":https") to specify the TLS port to listen on.
• https.selfsign (default: false) to enable the creation of self-signed certificates on-the-fly.
• https.organization (default: "ACME Inc.") organization name to add to the certificate.
• https.domains (default: "127.0.0.1") comma-separated list of names and IP addresses to add to the certificate. Could be "example.com, *.example.com" for example.

Changed config settings:

• https.enabled (default: false) is the new name for http.ssl (obsolete, but still works)
• https.certfile (default: "") is the new name for http.sslcert (obsolete, but still works)
• https.keyfile (default: "") is the new name for http.sslkey (obsolete, but still works)
• http.port is obsolete and will be removed in the future, please specify a full address string in http.addr and/or https.addr

API changes:

• mars.Run() does not take any arguments anymore, as this would not make sense in dual-stack mode.