Actually implement the majority classifier for comparison

[trevorcampbell]

From @joelostblom comment on Ch6: we talk about "majority classifier" but we never show how to create one. This is really easy to setup in sklearn and it is a useful strategy for students to learn so that they can compare against a baseline. I suggest we show them how to setup a dummy classifier (and a dummy regressor in later chapters). We might also consider renaming "majority classifier" to "dummy classifier" in the text so not be ambiguous