Rate limiter module for jobs

[robacarp]

This is a re-implementation of the throttling logic introduced in v0.4.0 via #20, but implemented as a decorator module.

The throttling configuration syntax is concise:

class RateLimitedJob < Mosquito::QueuedJob
  include Mosquito::RateLimiter

  throttle limit: 1, per: 10.seconds

  params blah : Int32

  def perform
    # etc...
  end
end

Highlights:

• Leverages before hook introduced in 7de390cf160756c16cbd360975786fb4fc698b66 (v0.11.1) so the interface is transparent, and doesn't have any performance penalty on jobs which don't need rate limiting.
• Leverages metadata store introduced in ae67378f9b7c5acaee60fb7df36816e26ea09a8c. The rate limit configuration logic is no longer dangling off of Queue.
• The rate limit key is now override-able, which means that multiple jobs can share the same rate limit counters. This hopefully will be useful for guarding against 3rd api usage guidelines.

Ancillary improvements:

• Implements after hooks, by copying and pasting the before hook code and tests.
• Extends metadata#increment to allow passing in an increment value.
• Fixes a latent bug where false could not be set as the default value for a job parameter.