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

@amZotti, @MikeyMurph77 - please take a look
the way we've been doing this in class is 
```ruby
"#{first_name} #{last_name}"
```
if you wan to use string addition it's more idiomatic to use single quotes:
```ruby
first_name + ' ' + last_name