Added RiakCS Support

Fixes issue #1100 

This is my first DD check, please feel free to speak of it harshly.

Tested in Centos 6.5 so far.
The self.warning method is to be used when something unexpected happened but didn't interrupt the whole execution of the check.

In the cases you are using it, it means that something bad happened and no metric will be returned. So you should probably raise an exception here instead (the exception will be caught later in the process and will be displayed in the info page).

Does it make sense ?