Record when payment is made for gigs

[marnen]

As a user I can enter and view date and amount of each payment a client makes to me for a gig So I can tell when a gig is paid in full

• [x] Put link to payment form on gig list
• [x] Save payments
• [x] Display payments on gig list
• [x] Put link to payment form on detail page
• [x] Display payments on detail page
• [x] Access controls: don't allow users to add payments for others' gigs
• [x] Tweak CSS
• [x] Edit existing payments
• [x] Final refactoring

[marnen]

UI idea: on gigs that haven't been paid in full, have a "received payment" button that makes a new payment defaulting to the current time.

[marnen]

Consider the money gem in order to make display of amounts uniform between contract and payment.