Improved API, Merged Various other Branches

Hi - I noticed that you created a fork of redis-mock and added it to npm as node-redis-mock. Since yours seems to be more actively maintained than the original I decided to make a pull request against your project.

I did two things:
- Redid the API (node-redis-mock.js) to comply better with the node_redis API. It now supports both methods of calling the functions as described at https://github.com/mranney/node_redis:
> All functions take either an args Array plus optional callback Function or a variable number of individual arguments followed by an optional callback.

- Merged various other branches. In addition to your transactions, there is now support for sorted sets, HMGET, LRANGE, RPOPLPUSH, and several server commands.