obsolete translations

added parsing for obsolete flagged translations (#~ msgid and #~ msgstr), so these get added to the translations. Obsolete translations have no msgstr / msgid, only comments.
Testing if #~ msgstr is present should be sufficient if lines are in usual order.

Added tests for this behavior. I had some failing mofile specs, but I think thats due to my ruby/gem environment and didn't bother looking into it as my specs pass, po parsing works fine and they also failed without any changes :).