Extra Utility method, modified webpart functions to work with web relative url

1) removed reference to obsolete method CreateSite and replaced it with CreateWeb
2) created url utility method to ensure trailing slash at the end of a url
3) modified the DeleteWebPart and AddWebPartToWikiPage functions to work with subsites. web.GetFileByServerRelativeUrl function was failing if only the page's relative url was passed. We need to pass the web server relative url + page relative url