Added relative method to Polish locale

[pepkin88]

Also deleted past, future and duration format strings, as they are now handled by the relative method.
Also added missing 3rd form for days dni and fixed singular form for month miesiąc.

I hope that it doesn't break backward compatibility. I was relying on the relative method from Russian locale.

I used the following test cases and for each case it sounds correct:

console.table([133, 60, 60, 24, 7, 4, 12, 30].reduce(function (ret, number, idx) {
    var unit = 'milliseconds seconds minutes hours days weeks months years'.split(' ')[idx]
    for (var i = 1; i <= number; i += 1) ret.push([
        i[unit]().duration('en'),
        i[unit]().duration('pl'),
        Date.create(i + ' ' + unit + ' ago').relative('pl'),
        Date.create(i + ' ' + unit + ' from now').relative('pl'),
    ])
    return ret
}, []))

[andrewplummer]

Hi... I've finally patched in your pull request here: eea0cca

Sorry for taking so long!

FYI I also got started writing the unit tests as well:
https://github.com/andrewplummer/Sugar/blob/master-2.0.0/test/tests/locales/pl.js

All you have to do is uncomment, fill in, and fix wording as necessary... If you're ever so inclined! :D