Support JSON-formatted job options for Sidekiq::Periodic::Loop when syncing periodic jobs

[tony-pizza]

Fixes #31 (see the excellent description there).

Fixes bug when syncing of Sidekiq periodic jobs due to cronitor-sidekiq expecting Sidekiq::Periodic::Loop job options to be a Ruby hash when they are actually a stringified JSON object.

• Adds some (minimal) testing
• Updates use of str.constantize to Object.const_get(str) to avoid dependency on activesupport
• Adds support for when Sidekiq::Periodic::Loop job options are formatted as stringified JSON
• Maintains support for when job options are plain old hashes (although I don't know if this is actually necessary)