Add make target to generate doc for internal API
Add a new module abstracting file writing

Instead of writing directly to disk, backend should return a list of
filepaths and content. This is good for several reasons:
- it makes testing easier
- it makes it easier to implement "output options" on a generic level
  (like change the output directory or printing a diff with existing
  files)
- it makes the code purer
Add a regression test for issue #30
Fix issue #30: XML printer follows -d option
Fix the Latex tests using the new backend type
Use writeFileRep in the new backend type too
Merge pull request #63 from gdetrez/issue#30

Fixe Issue#30: with option -d, generates Lang/XML.hs instead of XMLLang.hs