* Convert `_dependency_id` to `_dependency_ids`

Change `Job`s tracking from a single id of it's dependencies from a single _id_ to a list of _id_s. This change should be private to `Job` - especially leaving `Job#to_dict` and `Job#restore`s treatment of a single 'dependency_id' intact.

This change modifies existing tests.

* Remove reliance upon dependency property in tests

... use dependency.id not  `_dependency_id`

* Re-add assertions for Falsey Values

* Add _dependency_id property

For backwards compatibility with other libs such as django-rq and rq-scheduler

Without it you cannot specify custom timeout for failed jobs created using decorator

* modify zadd calls for redis-py 3.0

redis-py 3.0 changes the zadd interface that accepts a single
mapping argument that is expected to be a dict.
https://github.com/andymccurdy/redis-py#mset-msetnx-and-zadd

* change FailedQueue.push_job_id to always push a str

redis-py 3.0 does not attempt to cast values to str and is left
to the user.

* remove Redis connection patching

Since in redis-py 3.0, Redis == StrictRedis class, we no longer
need to patch _zadd and other methods.
Ref: https://github.com/rq/rq/pull/1016#issuecomment-441010847

This includes:

- a partial refactor of the CLI to organize the shared options
- extends the tests in areas where passing custom backend classes makes sense
- allow setting the core CLI options as env vars
- minor cosmetic changes here and there

Conflicts:
	rq/decorators.py
	tests/__init__.py
	tests/test_decorator.py