Rename SlotMachine\Page class to SlotMachine\SlotMachine

This was found to be confusing when importing the namespace, then using it as
new Page(), which a configuration file called slots.config, plus a different
context other than a web page may be possible for the SlotMachine library.

So this class will simply be renamed to SlotMachine.
Closes #9, new method to export all slots to JSON format
(WIP) Loosen dependency on Symfony HttpFoundation