Feature request: invoice view time (and IP?)

[lucaslcode]

Is your feature request related to a problem? Please describe.
If a client tries to claim they didn't receive the invoice, it's great to be able to tell them "My invoicing software shows the invoice was opened", but it's even more effective to say "My invoicing software shows the invoice was opened at 1:16pm on March 2nd". IP would be a bonus but maybe not necessary.

Describe the solution you'd like
The most robust would be an audit history of opens with times and IPs, but just tracking the first open might be sufficient and simpler to implement (just an extra column or two on the invoice table)