Add an option to include an API key for http://hostedgraphite.com

This adds support for using Hosted Graphite (http://hostedgraphite.com).

Basics are:

- Added a command-line option
- Added option to Server __init__
- Prepend all stats like ```APIKEY.<stat>```

Thoughts?

EDIT: Updated code to be more generic, use ```global-prefix``` option. Better for supporting multiple services and/or global-namespacing of stats.