Add support for external config file.

Attempt to load a config file, with increasing specificity int he following order:
- /etc/logbot.ini
- ~/.logbot/config
- command line args

This can be used to pass config to the bot as well as to override the listening address and port for the httpd service.

Add .gitignore for virtualenv files.
Add documentation for new config features.
Fix handling defaults for httpd when no config file exists.

This also standardises/improves the interface to getting items from a section.