l10n_moz_class: whitespaces in active tag, empty translations

* Issue #89: ignore trailing whitespaces in active tag.
* Issue #124: line following the reference string is not necessarily a
translation. In case of empty translation it could be a comment or the
next reference string.
Merge pull request #128 from flodolo/issue124

l10n_moz_class: whitespaces in active tag, empty translations