Fix XML Reporter to escape invalid characters and ensure valid output

Resolve #125 

- Update xml_reporter to XML encode attribute strings
    - Avoids problem where filename or description contains illegal XML characters (like an ampersand) by escaping the output
- Update README to use double-quotes in XML example (to match the updates to the XML reporter, since Ruby uses double-quotes for its XML-attribute string-encodings)
- Add XML chars to 'reason' in xml_reporter test to validate XML escaping
    - This test modification verifies that the XML-encoding of attributes in xml_reporter is working, since without that, the test would fail (due to problems parsing out attributes containing quotes)