Users can see who updated a performance last.

@christopherThorsen, @GarthGoldwater - Can you take a look at this?  
Steiner - the user_id had been set to string.  So when I tried to do what you are suggesting it gave me the following error when I tried to migrate:

20140813172905 ChangeUserIdColumnOnCustomers: migrating ====================
-- change_column(:customers, :user_id, :integer)
rake aborted!
StandardError: An error has occurred, this and all later migrations canceled:

PG::DatatypeMismatch: ERROR:  column "user_id" cannot be cast automatically to type integer
HINT:  Specify a USING expression to perform the conversion.
: ALTER TABLE "customers" ALTER COLUMN "user_id" TYPE integer/Users/andrewslawter/.gem/ruby/2.1.2/gems/activerecord-4.1.1/lib/active_record/con
nection_adapters/postgresql/database_statements.rb:128:in `async_exec'