Integrations overhaul

[nateberkopec]

As mentioned in #488, each integration should be setting more accurate culprits. A lot of the integrations could do with some general love as well.

I don't think we're clearing Context/Breadcrumb buffers everywhere either. Maybe just inline into capture?

• Sidekiq (Sidekiq: identify culprit + drop Sidekiq 2 #555)
• Rake (needs more context, take advantage of rake exception handling)
• DelayedJob (should be in our namespace, not DelayedJob's)
• RackTimeout (probably just a culprit addition)
• Rails (add culprit)
• Rack (cleanup "interface")