Add 2.8.0 new methods: scan/sscan/hscan/zscan

The new released redis 2.8.0 introduces 4 new commands for iterating over keys, sets, hashes and sorted sets.

Docs here: http://redis.io/commands/scan

I implemented these 4 methods, and wrote tests for them, see diff.

tested in 
2.7.6, 
3.3.0 (manually tested because I have problem with installing py.test in 3.3.0)