Implement API methods for campaign management.

This pull requests adds methods for campaign management (http://documentation.mailgun.com/api-campaigns.html), a feature recently added in Mailgun.

Implemented methods:
    - create campaign
    - list campaigns
    - get campaign info
    - update campaign
    - delete campaign

IMPORTANT: Because of a Mailgun API issue with content type (more here: http://bit.ly/1eF30fU) I had to add a hack in response handling. Basically lib now skips content type validation and assumes the response is in JSON format for Campaigns endpoint (only).