Fixes proxyLog 500 error case / better pathing

Logs are now relative to location of the proxy.config file - not the
.NET application directory.  Previous code was using
System.Web.Hosting.HostingEnvironment.ApplicationPhysicalPath where the
proxy.config may have been in a child folder.  This was not good.  Now
we look at the proxy.config location and if relative pathes are used
write there.