[FIX] tools.translate: when loading entries from a PO file, use also the POT.

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