Added Russian plurals support

- Locales now classes, based on single interface Locale. Now they have single method for humanization, but in future I'm sure there will be more
- Implemented locales for English and Russian (now with correct handling of Russian plural forms)
- Added more tests
Improve locales hierarchy

As major part of languages doesn't have complex pluralization logic — I've separated code for them to _BasicLocale class, so new locales can be defined simply by inheriting basic class and defining a dict
Added Ukrainian Locale

I wasn't sure about locale codes to use, but according to http://www.localeplanet.com/icu/uk-UA/ - locale code is uk_UA
Merge pull request #17 from cleg/master

Ukrainian locale added