Add support for Yandex.Metrika

[Yandex.Metrika](http://help.yandex.com/metrika/?id=1098577) is a tool, mostly analogous to Google Analytics, but maybe more useful for beginniers. It is being developed and supported by Yandex (Russian search engine company).

Widely used for websites with content in Russian language.

Would be very nice to see YaM included into Analytics.js.

(Tests included)