DATAREDIS-290 - Add support for SCAN command.

SCAN is currently only supported by jedis but can be emulated for
lettuce via eval.
Srp and JRedis will respond with UnsupportedOperationException.

We provide a Cursor implementation allowing to scroll through the
responses provided by the underlying connection. The cursor will fetch
additional results from redis server whenever its starting point has not
been reached and there are no more already loaded items available.

Only values returned by the last call to redis server are kept in memory
which allows scrolling through the entire collection without potentially
running into memory issues.

Original pull request: #71.