HL Python SDK: Alias Commit class to Reference

[N-o-Z]

Currently Commit class is an data model containing commit data. It can also be used an alias for Reference and can then be used seamlessly to provide reference functionality when received as a return value (from log method for example)

Issues to solve: Currently Reference object has the method get_commit() which returns the commit data for the reference in a Commit object. We might want to change that so we don't have the following syntax: commit.get_commit()