Simplify entrypoint

[fxn]

Hey! :)

The autoload calls in this file are unnecessary, because the require calls above already define those constants.

Since I was on it, I refactored the way I normally write a file like this. Let me explain:

• Since this file is simple_calendar.rb, I find ordered that it owns the creation of the SimpleCalendar module. Then, descendants reopen. They can also use constant paths like SimpleCalendar::ViewHelpers in the definition of child classes and modules if they so wish, because they can rely on the parent module being already defined upwards. (This is only a consequence, not the motivation.)
• For the same price, we can avoid $LOAD_PATH lookups by using require_relative.

However, these points are kind of personal style, if you prefer something else please just tell me :).

[excid3]

Thanks @fxn! This all makes sense to me. Appreciate it!