Save the user who returns a loan

If a user is provided to the Loan#return method, save the user as the 
'returned_by' relation.
Change Copy#return to take optional 'as_user' arg

This also removes the constraint for the returning user to be the same
as the user who borrowed the book.
Allow a copy to be returned by any user
Display the returning user in the history list

If the copy has been returned by a different user to the user that 
borrowed it, then display the returning user's name in the list too.
Merge pull request #15 from JordanHatch/anyone-can-borrow-return

Allow anyone to return copies