Allow specification of "@fatal_exceptions"

When a fatal exception is raised the job will be immediately marked as
"failed" (this is the inverse of "@retry_exceptions").

High level changes:

* Ensure that only one of the two exception handling stategies is
  specified (e.g. "@fatal_exceptions" or "@retry_exceptions")
* Favor checking "retry_exceptions" over "fatal_exceptions" as it has
  been part of the API for a longer time
* Add tests to cover the validation and feature behavior
* Update documentation
Introduce a little randomness into "retry_delay"

Add the ability to specify either "retry_delay_multiplicand_min" or
"retry_delay_multiplicand_max" or both. These values will be used by
the "ExponentialBackoff" retry-strategy. By tuning these values you
can stagger retry attempts in situations where a large number of jobs
fail around the same time (e.g. rate-limiting/throttling or connectivity
issues).
Update README.
Validations for multiplicand min/max configuration

Ensure that "retry_delay_multiplicand_min" is less than or equal to
"retry_delay_multiplicand_max" (this applies to cases where both values
are specified or if only one or the other is specified and it conflicts
with the other multiplicand's default value)
Adjust example code to start redis-server.
- Do not swallow exceptions caused by missing job code in resque web interface.
- Adjusted retry tab code to correctly `constantize` job class.
Version 1.1.4
Get the retry_attempt for DirtyExit failures, where perform isn't called.

@retry_attempt depends on perform being called, but perform will be called
after forking and set in the child job process. If there is a crash that
prevents the job from notifying the failure hooks itself, then a DirtyExit
exception will be given to the on_failure hooks from a worker processes,
where @retry_attempt will not have been set.

The @on_failure_retry_hook_already_called variable also needs to be
ignored, because this is set to false in the before_perform hook.
Add test for retry attempt being accurate for dirty exit failures.
set a TTL on the retry keys to make sure they dont stick around