DotNet feature log messages to disk

If logFile property is present in the config file, the DotNet proxy will
write log messages to disk
Support relative path in logFile
Support dot slash relative path logs / remove relic
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.
Merge pull request #141 from phpmaps/master

DotNet: log based on proxy.config