Fixed nchurn to the latest style.

I like what nchurn does with the parameter setting methods, it's way cleaner
than the custom metaprogrammed attr_hash and attr_array. I'd like to standardize
on this style! But, the attr_accessors have momentum. So, for just this one
task, and because I don't know how well used it is, I'm supporting _both_
styles.

The only trouble with the method style here was that the literal properties,
assigned by the rakefile/user, were being modified _on assignment_. I'd rather
leave that until the cmdline string/array is being built up.