Redis: provide a generic interface, import redigo optionally from main

[flashmob]

issue #94

A new interface has been created for Redis (redis_generic.go) complete with a mock implementation.
A driver is in a different package, in backends/storage/redigo/driver.go
This driver is enabled from serve.go found in the cmd/guerrillad dir, similar to the way the mysql driver is enabled

[jasonfriedland]

Hi, what's the timeline for this being merged?

I was about to submit a change to update the redigo import path to fix internal/ package import issues from another package which has the same dependency but is updated to use the new path.

This change will fix this :-)