[ADD] orm: add the possibility to export in raw format instead of all string
[FIX] export: negation issue
[FIX] convert datetime to date in case of date field
[ADD] orm: Allow to export data in raw format instead of all string.

This is useful for exports in MS Excel format which needs raw data to handle correctly the number and dates.
Merge branch 'master' of bzr::lp:openobject-server