Simplified and improved IIS URL Rewrite rules in web.config, also fixes #3899

Removed unnecessary/duplicated rules.
Added rewriting for text.php
Added exclusion of direct access to app/webroot/*
Removed unnecessary condition tags
Added rule descriptions
Removed url={R:1} from last IIS URL Rewrite rule
Merge pull request #619 from ravage84/iis-url-rewriting-improvements

Simplified and improved IIS URL Rewrite rules in web.config, also fixes ...