fixed po to json to po conversion: context, english plurals, russian examples

This pull request fixes po -> JSON -> po conversion when there are msgctxt tags are used in the po file. 

The original implementation changes msgid to the `originalName_Context`. This pull request adds context name into the key name in JSON but trim it from po.

There was also a problem with plurals order during json to po translation: English followed French branch because of setups in the plurals.rules. I simply dropped that two branches.. I don't know if this solution is correct, but at least it works for me.

Thanks