Documentation for constants used by ``query_string.create_query`` 

Branch forked off of ajl/docstr-issue-5: 

I'm heavily on the fence about whether we should care to include the documentation for these keys.  But, they are mentioned in the API specification document: 

http://support.kissmetrics.com/apis/specifications

For completeness, I have broken them out into this PR so that they can be included or ignored - whichever is most appropriate.