Fix for invalid XML characters in Log4JXmlEventLayoutRenderer

Added a helper class for handling invalid XML characters (Use in Log4JXmlEventLayoutRenderer)

I'm using a external obfuscated dll.. They used obfuscated method names which contain invalid XML characters. The XmlWriter methods in Log4JXmlEventLayoutRenderer could not handle them.

![image](https://cloud.githubusercontent.com/assets/5426370/2852781/27ccdd64-d13b-11e3-8ac8-72640b39d02a.png)

I created extension methods which filters the invalid XML-characters..