Fix/xml escape

partial fixes for #24 

- escape some XML when writing mzXML and mzML
- try to fix the worst offenders
- test for some attributes in mzML
well this assumes that characters that are invalid in XML are also invalid in OBO. This is a strong assumption and from the specs I cannot really deduce that : http://geneontology.org/GO.format.obo-1_2.shtml#S.1.5 the escaped characters are clearly different