Isolate Sidekiq's own context info into :extra.

Sidekiq sometimes sends a :context key as part of its error handling
context hash. However, it means something entirely different than
Raven's own :context key in options. Move all of Sidekiq's exception
context into :extra so it doesn't confuse Raven.
Merge pull request #199 from zarqman/fix_sidekiq_errors

Isolate Sidekiq's own context info into :extra