Support 'Account-Email' header for multi-user

[smeyfroi]

Adds an 'account_email' attribute.

Might address #8 ?

Use like:

    pingdom_check probename do
      api_key node[:application_geocms][:pingdom_api_key]
      username node[:application_geocms][:pingdom_username]
      password node[:application_geocms][:pingdom_password]
      account_email node[:application_geocms][:pingdom_account_email]
      host probe["host"]
      type "http"
      check_params(
        url: probe["url"],
        encryption: true,
        port: 443,
        shouldcontain: probe["shouldcontain"],
        tags: [
          "#{node[:application_geocms][:project_name]}#{node[:application_geocms][:environment]}",
          node[:application_geocms][:environment],
        ].join(",")
      )
    end