type checking for contact_points in Cluster constructor

I recently fell prey to passing in a string as the contact_points variable, which led to an error only after calling cluster.connect(), which happens in a for loop that incorrectly attempts connect to each character of the contact_points string. contact_points is expected to be a list or a tuple. I think it would be useful to catch this sort of simple error inside the Cluster constructor, as opposed to delaying the error or hoping that it gets caught when connecting.


P.S. This is my first attempt at contributing to any real open source project. :) Let me know if I am doing something wrong, and I will try to fix it.