Add support for supplying Procs for rate limit, name, and period.
Add docs for dynamic configuration options.
Fix a couple of README typos.
Handle when Sidekiq::BasicFetch.retrieve_work returns nil.

Fixes a crash caused by attempting to analyse the work’s message when the work is nil.
Fix incorrect behaviour due to too much cached state in Sidekiq::RateLimiter::Fetch.

Fetch was storing memoized rate data across jobs, causing rates specified for one job to be applied to another. Fixed this by factoring out a Rate class that can be used to store memoized rate data for each job that Fetch processes, before being discarded.
Fix procs being ignored due to non-stringified keys.
Merge pull request #7 from avalancheau/procs

Add support for Procs in configuration options.