unique translation check

the unique translations check was flawed.

According to gnu gettext reference (http://www.gnu.org/software/gettext/manual/gettext.html#PO-Files) a translation is unique if the combination of msgstr and msgctxt is unique. The old method ignored the msgctxt and removed a lot of unique translations. I fixed that.

Additionally i implemented an optinal merge argument, that allows to merge References of non-unique translations, so instead of deleting all but the last translation, we keep the last and merge all other references into the last. This isn't perfect, as only comments and flags (fuzzy) of the last translations remain, but imho good enough for an option that defaults to false.

specs are included. A new gem version would be appreciated if you agree with the changes.