Added ability to add campaign ID(s) to a message

This release adds the ability to add one or multiple campaign ID's to a message using the new campaign method.

Requested by @GrandadEvans in #19.

---

This method accepts a single ID or an array of ID's with a maximum of 3 ID's (limit is set by Mailgun). If more than 3 ID's are provided Mailgun will automatically omit the remaining ID's.

$campaigns = array(
    'campaign-ID-1',
    'campaign-ID-2',
    'campaign-ID-3'
);
$message->campaign($campaigns);

or

$message->campaign('campaign-ID');

I'll start tagging versions from now on. I previously messed the version numbers up a bit because of the compatibility issues with Laravel 4.1 and 4.0.
I'm going to resume from where I left off (v2.0)