Added sendgrid ip pool capability

[vladshub]

This is a rebased PR #20

[a17levine]

+1 (I could use this)

[a17levine]

UPDATE: Nevermind this didn't work. I ended up just forking your repo!

I ended up using this code in the meantime in the mailer method

    headers['X-SMTPAPI'] = { 
      ip_pool: "not_trusted_mail" 
    }.to_json