Exposing Webternet functionality as natives

This patch exposes the HTTP logic of the Webternet extension as SourcePawn natives. 
See `plugins/include/webternet.inc` for a list of well commented natives. 

A sample plugin demonstrating the usage can be found [here](http://www.xup.to/dl,90855285/webtest.sp/).

I also created/adapted a VS 2010 project and solution file for my local debugging; it's not needed for an official build of course and my be scraped. I put it under source control to make things easier.

If you are wondering why there are so few commits; I created this with the "old" mercurial repository clone and migrated it locally to git.
While the thread pool does indeed default to a single worker thread, you can spin up more with SetMaxThreadsPerFrame.