Add sum and sumfloat commands to sum keys. 

I suggest to add this usefull(at least for us) and simple command. It just returns a sum of a bunch of numeric keys. All non-integer (non-numeric for sumfloat) and non-existing keys are ignored. This behaviour is discussable (for example we can return an error if there is at least one non-numeric or non-integer key), but I personally tried to be consistent with other commands (for example mget command will return nil for non-string key). I would be glad to make a pull request to documentation if you find it usefull too. Many thanks.