Accepts a JSON string of an email address (from Zapier, most likely):
{ "email": "nathaniel@ticketbud.com" }
If the contact doesn't exist, we create it, and tag it with "Unbounce" and the currrent time.
If the contact does exist, we tag Lead_Source_Unbounce with the current time. If the contact doesn't already have a Lead Source, we go ahead and add Unbounce.
Would probably need a bit of hacking to get working with your setup, but hopefully serves as a good starting point.