Add some configuration settings for Twitter.

Any configuration settings in a file called private.config will override the
settings in app.config. This means we don't have to check in things such as the
API secret keys to GitHub.
Always copy private.config to the build output.
Add command line option to authorise a Twitter account
Remove some config settings that we're not going to use.

The user key and secret are written to private.config by the application,
so don't store them in the existing config file.
Tweet it!