ConvoKit version 2.0 public release

This is the public release of the brand-new, overhauled ConvoKit API, marking a major version number bump to 2.0.

Compared to previous releases, the newly refactored API has been heavily streamlined to unite all conversational analysis modules under a single consistent interface, which should hopefully decrease the learning curve for the toolkit. The new API is inspired by scikit-learn and should be familiar to those who have prior experience with that package. A high-level explanation of the API and object model can be found here along with a step-by-step tutorial for getting started programming with ConvoKit.