Huge Refactor

• Client and server are now in the porthos (top level) package.
• Server now has a blocking ListenAndServe function.
• Server now has Close (async shutdown) and Shutdown (blocking shutdown).
• Server now has RegisterWithSpec function to provide a spec alongside with the handler function.
• Added a built-in extension to ship (to the playground app) all registered procedures with specs.
• Status code constants were renamed after moved to the top level package (porthos).