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
Also, there are some obo terms that do have special characters at least in the description (fortunately not in the name but who know what will be next):
```
[Term]
id: MS:1001993
name: quick and dirty MALDI sample preparation
def: "Quick & dirty (Q&D) sample preparation separating matrix handling from sample handling." [PSI:MS]
is_a: MS:1000833 ! matrix application type
[Term]
id: MS:1002480
name: regular expression for a digital object identifier (DOI)
def: "(10[.][0-9]\{4,\}(?:[.][0-9]+)*/(?:(?![\"&\'<>])\\S)+)." [PSI:PI, http://dx.doi.org/]
is_a: MS:1002479 ! regular expression

```
ok - who puts a regex into a structured document with an identifier to it? Whose idea was that? We should really start a blog about a computational mass spectrometrist and his descent into madness. Come on ... 

I would love to see how that looks when XML-encoded :-)