Translating the Python lesson on conditionals to MATLAB

This is an early pull request to resolve #708. There are a few FIXMES, but I think an early review might help.
Maybe add

~~~
if true
    disp('variable true')
end
~~~

**Disclaimer**: I don't know if the `true` variable is GNU Octave specific:

~~~
octave:1> true
ans =  1
~~~

And it has the same behavior of `True` at Python, i.e., "is just a name that exists in the scope by default.":

~~~
octave:2> true = 100
true =  100
octave:3> true
true =  100
~~~