Support passing port as string to HttpServer

According to the mongoose documentation, any port on which you want to enable SSL must have an s appended to it. Currently, the port is passed as an integer to the HttpServer constructor despite the fact that the mongoose API expects a c string.

I added a second constructor that takes a string for the port . This allows one to pass a comma-delimited list of ports and to specify which are to use ssl. Furthermore, the port is now a string member of the class. 

For backwards compatibility, the original constructor prototype is kept as well.

// example
HttpServer server("80,443s", true, "my-cert.pem");