add support for Account-Email header

If your pingdom account has multiple users you can auth to the API as your account, but you have to send a special header with the email address of the account owner.  Pingdom's docs say this is a feature...

Docs: https://www.pingdom.com/features/api/documentation/#multi-user+authentication

Pull request adds support for this header in the main constructor so connecting is still 1 line of code.  I'm using the modified library in my project and it works as expected.