Go weekly 2012-02-07.

Example is now capexample.
To build helper tools:

   cd capexample && go install
   cd capgen && go install

This will install tools capexample and cangen into $GOPATH/bin.

sounds.go doesn't have a copyright header, because the file is generated
by a program. I don't want to make it output the copyright header,
because users can generate sounds themselves, and the copyright status
of such generated files can be different.

NewLen is now like the old New (accepts captcha length as the argument).
Also, rename StdLength to DefaultLen.

Only return page for root path /, return 404 for any other, such as
/favicon.ico.