added a gitignore
made some changes

GitHub won't let me divide up these changes for some reason (perhaps a
bug), so here's a quick summary of what I did:
- added some constants
- made some formatting in the global variables section
- added some variables
- tried to make the monsters die when out of bounds
- made a monster generator correctly
- tried to make carrots immovable and also die when out of bounds
- made a generate monster method, a gravitate to player method, and a
kill monster method