On Launchpad, as commented on bug 933496, PO entries (and their comments) are
shared between series. This mean that e.g. the 7.0 series can have the wrong
`reference` comments (those beginning with #:) as they would be copied from say
the trunk series. Those `reference` comments are used to import translations
and look them up.

This patch adds a few lines of code to tools.translate so that targets
defined in the POT `reference` comments are used in addition to those from the
PO file.

Also adds a test module to validate the new behavior.

This patch stems from:
 - the 6.1 branch by Vo Minh Thu:
    https://code.launchpad.net/+branch/~openerp-dev/openobject-server/6.1-fix-po-targets-933496-vmt
 - the 7.0 port by Numerigraphe:
    https://code.launchpad.net/~numerigraphe-team/openobject-server/7.0-fix-po-targets-933496-vmt