Allow building multiple language packs in one run

Fixes bug 2999
Remove the check that source and target are the same

Fixes bug 2805.
Add unit test for the previous commit
Add --removeuntranslated option to po2dtd

See bug 1718
Add --threshold option to po2* converters

An option to specify the percent of translated words below it no file is
output.

See bug 2998
Move the conditional to should_output_store()
Add --removeuntranslated to po2moz

Also pass it through and prop2mozfunny converters.