lcase rest keys, prefix constructor args

Thanks for creating this cfc! Minor changes summarized below:

key -> lcase(key)
 - could only get sendMessage to work with this change
 - referenced here: http://stackoverflow.com/questions/22200262/mailgun-cfmailgun-component-how-to-implement-it

apiKey -> mailgunApiKey
 - with dependency injection tools, such as Sean Corfield's di/1, having more specific constructor arguments makes conflicts less likely to occur with other application wide settings (which are automatically injected when creating the cfc instance)