Support multiple versions of MediaWiki

[zverok]

Many popular sites have MediaWiki version lower than latest. Some of them even as old as 1.19 (of 2012!)
Differences between versions can both in available modules and core logic (like token management)

Ideal support for multiple version should consist of:

• multiple generated modules lists, like lib/generated/<version>/*.rb (currently, code is generated from online help at mediawiki.org/w/api.php?action=help; for multiple versions it should be generated from MediaWiki sources, using tags?)
• multiple clients, like MediaWiki::Client_v1_19, loading different modules
• transparent API version discovery on client creation.

Sub-ideal solution should at least tag modules with minimal version they require, and warn user on attempt to use those modules on wikis with lower version.

NB: In older versions of MediaWiki (even in slightly older ones) query continuation logic is much more complex and inconsistent than now.