HttpServer: Retrieve secret key from request data instead of headers

The request data is already expected to be in json form, so the secret
key can be retrieved from the body instead of the headers.
vmnetx-server: Add options to run the HTTP server at defined host and port

The HTTP server by default runs on localhost on a predetermined port. The
option to define the host and port of the HTTP server makes it possible to
control the server externally.
vmnetx-server: Add simple test client

The new options allow the script to attempt to create a token at a given
server.
vmnetx-server: Enable threading for the HTTP server

The HTTP server should be able to handle multiple requests simultaneously,
so enable threading.