Test for Redis versions < 2.4 before storing arrays

SADD only supports multiple member arguments since version 2.4, see
http://redis.io/commands/sadd. The last Ubuntu LTS still installs 2.2
per default.

Also I factored the Redis functionality of the base class into it's
own class (but the reporter still uses redis directly) and did
other small refactorings.
Merge pull request #4 from iGEL/redis_fix

Test for Redis versions < 2.4 before storing arrays