Created ReferralCode

Moved the the code from Referral to ReferralCode. Having them on the Referral model
just seemed wrong.
Created ReferralCode

Moved the code to the new model. Having the code on the Referral would have caused issues
since the code would have been a one time use the way it was setup.
Merge pull request #5 from foxio/referral_code_table

Referral code table