Add support for $_GET and $_POST

Took the params off the env and injected them before any PHP pre-processing making them available right away. All the parsing relies on PHP's [parse_str](http://www.php.net/manual/en/function.parse-str.php) function and works even for multilevel arrays.

Also added the Cache-Control header as a precaution because I'm not completely sure about the Rack caching internals.