Modified to allow for hooks into the code

This pull request updates a lot of Gorilla REPL to allow for an external system to hook into Gorilla REPL.

There are options for overriding fixed strings in the client by sending configuration information (e.g., Title).

The `worksheet.html` file can live at a different context path and all the services are relative to this context path (e.g., `/worksheet.html` becomes `/foo/bar/worksheet.html`)

Resources (e.g., webservices URLs, MathJax, etc.) are accessed securely if the original page was served via HTTPS.

There's a 1 second ping between the client and server (over the websocket) to keep the web socket alive across proxies.