discard logging in webgo framework.

This creates a logger with output to a /dev/null equivalent. You still pay
the price for formatting and copying, but there will be no IO or write
syscalls caused by logging.

refs #253

If you also like to disable the logging the standard library does
sometimes, the add `log.SetOutput(ioutil.Discard)` too.

Please note, that such programs are very hard to analyse then!
If you want me to make it an option, please say so.

@bhauer please take a look.