Support creating campaigns via the API

Thanks, really useful library.

I need the ability to create campaigns so I've patched the module and though I'd contribute the patch back in case you wanted to incorporate it back into the module.

I've added these method to CampaignMonitor class:

getDraftCampaigns
getCampaign
getTemplates
createCampaign
deleteCampaign
updateCampaign

I've also added another 2 tabs to the Campaign Monitor settings: Campaigns and Templates.

Note to actually create a campaign another module is required to call the additional methods in the CampaignMonitor class. I've not created a whole UI to create and send campaigns.