Add new bulk push command

[mperham]

Add PUSHB for pushing many jobs with one network call.

PUSHB json
> PUSHB [{job},{job},{job},...]
{}
> PUSHB [{job},{job},{job},...]
{"123456789": "Invalid something or other"}

PUSHB takes a JSON-formatted Array of job Hashes. The return value is a JSON-formatted Hash of JID: Error pairs, empty if all jobs were pushed successfully.

[happycappie]

We rely heavily on push_bulk with Sidekiq currently and it would be most useful to have similar support when using faktory. Our goal is to push large batches (10s or 100s of thousands of jobs) from our Rails app to an elixir microservice.