Make MultiJson optional for Ruby 1.9+

MultiJson is being EOL'd and major libraries (Rails, uglifier, etc.) have started phasing out their dependency - https://github.com/rails/rails/pull/10576 .  This PR accommodates this trend, by making MultiJson an optional dependency for Ruby 1.9+.

Users of the jwt gem can still use MultiJson even when using Ruby 1.9+ by simply calling `require 'multi_json'` before they require the gem.