Update to work with multiple Campaign Monitor accounts (API keys)

[lylo]

Previously, List, Client and Campaign objects generated a new CampaignMonitor client object in their initialize methods. This defaults to using the system-wide CAMPAIGN_MONITOR_API_KEY constant, which prevents using multiple API keys if you have to add data to two separate Campaign Monitor accounts in your app.

This change allows a pre-instantiated CampaignMonitor object to be passed into List, Client and Campaign, thus making working with multiple CM accounts trivial.