Introduce the guess my number game.

You can play from irb as follows:

```
$ irb
irb> load './guess_my_number.rb'
```

You start a game with the command: `guess_my_number`

You play the game using the commands: `higher`, `lower`
Activate initial guess test
Create NumberGuesser class
We can make a guess!
Activate lower test
Guess lower than before
Activate higher spec
Guess higher than before
Activate guessing down to 1 spec
Make guessing lower more efficient.
Activate going up to 100 spec
Apply the same principle to guessing higher
Activate edge case specs
Use method names to document how the guess works
Don't return useless values from command methods.