Don't require @ prefix for attachment filenames

After using `Mailgun::sendMessage()` directly and attaching files via the `$postFiles` parameter, I was surprised to get an `InvalidParameter` exception when passing identical parameters into `MessageBuilder::addAttachment()`. 

I'm guessing there was a reason to require the "@" prefix, but I don't know what it is, and everything that I tried works without it. Moreover, `Guzzle\Http\Message\PostFile::setFilename()` simply strips the "@" first thing without doing anything with it. (See https://github.com/guzzle/guzzle3/blob/master/src/Guzzle/Http/Message/PostFile.php#L48).

This PR is to make `addAttachment()` behave consistently with `$postFiles` by not requiring the "@" prefix.