Implement create performance with a related cause

[aonomike]

What does this PR do?

• Ensures performance can be created with artists
• Implements creating performance with cause

How should this be manually tested?

1. On terminal, run command ...

mix test

What are the relevant Github Issues ?

Fixes #142

[yakryder]

Thank you Mike! Build failing

 1) test create_performance/1 creates a performance with a related cause (SingForNeeds.PerformancesTest)
     test/sing_for_needs/performances/performances_test.exs:47
     Assertion with == failed
     code:  assert performance.title() == valid_attrs_with_cause_id.title()
     left:  nil
     right: "Awesome Performance A"
     stacktrace:
       test/sing_for_needs/performances/performances_test.exs:54: (test)