xhr() function was refactored

This change implements next features:
1) Deal with "access is denied" error in IE 7/8 if script is using in local files (demo example doesn't work in IE 7/8, only IE6). Thanks to CSSUtilities plugin (http://www.brothercake.com/scripts/cssutilities) for information inside source code
2) Code optimization: using ActiveXObject instead of XMLHttpRequest for IE 7/8 allowed to skip browser check and use closure without "new" keyword which is equal to all other browsers
3) getXMLHttpRequest was highly optimized and moved to xhr() method due its small size