Use pgettext to translate number.ordinal

The problem is that, eg, Russian language have different ordinal
suffixes:
N	en	ru
------------------
0	th	ой
1	st	ый
2	nd	ой
3	rd	ий
4	th	ый
5 	th	ый
6	th	ой

So, there is no direct mapping. This fixes problem particularly, but
there is still some problems with values > 10, but I don't know
how to fix them yet.
Make `i18n.activate` idempotent call.
Fix for alternative 'if' branch
Merge pull request #17 from seriyps/pgettext-ordinal

Use pgettext for number.ordinal