Initial pass at Yandex.Translate support

This works in a similar fashion to Google Translate, except with fewer
language options and no variants. It has a similar limitation in the
length of phrases that can be supported, so perhaps those things can be
fixed together.
Quality support for Yandex.Translate

This makes the quality parameter configurable by the user. Only 'hi' and
'lo' are available.
Update README to include Yandex.Translate

when going to stable, the Anki add-on page will also need updating. This
service will be the second service available in the add-on that will
always be turned on, no matter what platform you are using.
More conservative throttling defaults

With a new online service, the add-on should ship with very conservative
throttling defaults. As such, this changes the threshold/sleep ratio to
be 1:3, also having it kick in more often.

Users can tweak this setting, if they wish.
Merge branch 'add/yandex-23' into develop

Brings in support for Yandex.Translate as an online service. As a
side-effect, this does increase the conservativeness of the online
service throttling, although a user may adjust it to be more aggressive
if they like.

Closes #23.

This service is affected by the same limitation as Google Translate as
discussed by issue #21, but they will get fixed together.