Allow for optional console password.

Added the ability to enforce cursory password protection for rack-webconsole.

To enable the console password, add something like this to your project's environment:

Rack::Webconsole.console_password = some_method_to_retrieve_your_password_from_config

The user's application should store the console password in a config file (hopefully encrypted) and use the above method to set the console_password.

If you think we should take this a step further and require the password to be stored as a hash within rack-webconsole, then we can do so, just let me know.

I removed the previous pull request and consolidated the changes per our discussion earlier.

Thanks!