Delete data from Redis when object is deleted.

[esbanarango]

When we delete an object all the tracking activity space on Redis should be removed too.

[JonMidhir]

Thanks for raising this and taking time to submit a PR. I agree that this should be introduced to Redcrumbs but I'm not sure that it should be the default behaviour for two reasons:

1. Unlike, say, a versioning system - where a deleted object can really have no versions - Redcrumbs is about leaving a trail of activity behind an object which may persist in the world after the object has gone.
2. It changes current default behaviour, potentially disrupting peoples' expectations.

Therefore, I think this should be opt-in behaviour, probably via an option when an object is redcrumbed, similar to :dependent => :destroy on ActiveRecord associations.

I'll leave a couple of technical notes on the PR. What do you think?