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
I'd add a scope to `Contact` so you can do `contacts.customer`. Same for the similar methods below