[FIX] Error when manually adding analytic account in the generated tax lines on an invoice

fixes #374
fixes https://bugs.launchpad.net/ocb-addons/+bug/1084822

The fix considers invoice tax lines with different analytic account
are equivalent for the purpose of checking if the list of tax line
is complete.

Caveat, this changes the structure of keys in the dictionary
returned by account.invoice.tax's compute method, I suppose this
is ok for the master branch.