Various improvments.

4c75e6f Using `#!/usr/bin/env python` is more portable.
8da76d2 Avoid having a bunch of saved code added with commits.
7a63cd1 Added list of non-default Python modules required to README. Good to know, as I had to install them before WhileyWeb would run.
2fac4f8 Santizes the `id` parameter by removing any character that is not a letter, number, or hyphen.
23bd043 If the `id` is invalid, it displays an error for 5 seconds then redirects back to the main page. This also includes the foundation for other (non Whiley/Java) errors to be displayed to the client, with an optional redirect.
f41e358 Display the HelloWorld program if the id parameter is not given.