Try to make the log directory if it doesn't exist

This patch makes Yaws try to create the log directory if it doesn't exist.  Pretty self explanatory.  It's useful when doing application testing, where I can spin up many instances of Yaws in different potentially unknown base directories, without having to figure out how to make the log directory first.

I based this patch off of the yaws-1.96 tag for my own uses, but it applies cleanly to the current HEAD.