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.
I don't think so, because Riak CS *acts* like a proxy as a presentation of it's service for clients ( i.e. boto ) who are used to connecting straight to s3.amazonaws.com. So this shouldn't be the same as any global proxy configuration the agent might have.

I have re-worked the settings a little so it's less confusing. Riak CS can be configured as the root s3 url ( i.e. s3.yourdomain.com ), or accessed as a proxy to s3.amazonaws.com. If it's the root url, it can still be accessed as if it's a proxy, but the root url needs to match. I think the changes I made make it more clear from a configuration stand point.