JSON API improvements

[mwielgoszewski]

Following PR adds the following functionality:

• Updates the listResponseElement struct to include the fully marshalled Certificate object, Rejected status, request Environment, and request Reason.
• For HTTP responses that return JSON data, set the Content-Type header to reflect this.
• Add a command line boolean switch (and environment variable) to runserver to support running ssh-cert-authority behind a reverse proxy such as Nginx. When running in reverse-proxy mode, Gorilla's ProxyHeaders middleware is used to inspect X-Forwarded-For and/or X-Real-IP headers to update the remote address on the request object accordingly (along with other information).

[bobveznat]

All merged in. I also updated the readme to talk about this and the other flags.