Add Gemfile
Add notes

This is a file of notes I used to have on my machine.
Now that John is also working on the project, this should be shared.
Fix dynamic behaviour of defined words

Now defining a word works like in Gforth:
it gets "compiled" the moment you define it.
Fix style here and there
Share state without all the ugliness

The interpreter now shares state with the AST classes
without using globals.
Separate UndefinedWords and Integers in two different classes
Raise error when defining a word with undefined words