Split commands into different files & add bitcount

First, I split all the commands into different files. Each file match a redis commands category. Also, I add comments to know for each category which commands are implemented and which are not (redis version 2.8.2).

The implemented methods have the same name as the corresponding command, but when you want to edit that file it can be a little messy, so I thought that would be ok to organize the files in that way. Hope you like it.

Now bitcount is implemented (specs added too). It would be great if you can take a look on it.

Things can be improved:

* Docs. I know the methods are clear because its name, but I think YARD or RDOC documentation would be a good added value to the gem.
* Decouple into more modules. 
* Use concerns to handle dependencies
* Constants. Everywhere is used 'OK'
* Exceptions for unimplemented methods (when I have the problem with bitcount it wasn't obvious for me to think that the gem doesn't implement that command). Add those exceptions also avoid the documentation to know which methods are not implemented (still it can be kept as a summary) and the placeholders acts as a good help for developers.
* Group specs. I do that for bitcount, if you like the example it can be used as an example. Is hard to read which methods have specs.

Sorry about english, and thanks for the gem.
Regards