Translation docs + code improvements and fixes

* Start of module translation guide for developers
* Export language-less PO file as POT
* Remove unnecessary default in export wizard
* Improve export wizard code (leverage browse record, ensure output buffer correctly closed, …)
* Fix a pair of incorrect translation marks
* Fix export bug (ignoring module of first line of language export), remove dead code and simplify some stuff
* Convert recursive view parsing/extraction to iterative via [Element.iter](http://lxml.de/api/lxml.etree._Element-class.html#iter)

@rco-odoo since you'll apparently work on translations stuff I'll leave this branch to you (if you want to review/use/merge it)