Improve stats-po.sh script

Besides style changes (use consistently ```$(expression)``` instead of ``` `expression` ```, the current script suffers from a series of issues: it counts ```msgid```, but that means that it always increases the real number by 1 when it's not 0 (there's an empty msgid), and counts plural forms as missing strings (```msgid_plurals```)

Current script
```
./sr_Latn       main=304    js=5
```

New script
```
./sr_Latn       main=297    js=4
```

Which is correct compared to msgfmt

$ msgfmt --statistics sr_Latn/LC_MESSAGES/javascript.po -o /dev/null
128 translated messages, 4 untranslated messages.

$ msgfmt --statistics sr_Latn/LC_MESSAGES/messages.po -o /dev/null
1543 translated messages, 297 untranslated messages.