Arg 20130421

Move Controller::arg_request() to Request::arg(), add "optional" argument.
- K3 controller actions neatly take things from request (param, query, post, url)
and put them in response (headers, body, send_file), so arg should follow suit
instead of being in the controller object.
- arg_required() is used 92 times and arg() just 1 time, so the syntax should favor
the much more common case.  arg_required() is now called arg(), which can be used
as arg(0, "optional", "default") when rarely needed.

Total usage count:
5 (no rule)
74 digit
6 alpha
7 alpha_dash
1 optional

Thoughts?