Create and view performances

This feature turned out to be several:
*Create a performance
*View a performance
*View the date create and by whom
*Add multiple customers and multiple partners to a performance
*View all customers and partners associated with a performance
*View a list of all performances by customer

@ChristopherThorsen - please take a look
This isn't gaining you a anything, really - I'd just do `@customer.contacts` directly in the view.

Bonus: fewer instance variables to set.