Add upper limit and regeneration to MP

- Player.max_mp has the upper limit for Player.mp.
- Player.head.userData['update'] regenerates MP now.
Add my email address
Simplify gitignore

- Also I set core.excludesfile.
- Is it too simple?
Add items with bugs

- Doragon's some bodies drop items when it's destroyed.
- Pick up an item with 'e' key.
- The position which item is dropped at has a bug (enbugged). It is not
  always same to the body's position.
Improve player's stability

- Avoid swinging around the center of mass.
Rename 'destroyed' 'on_destroying'

- Actually, it is called before DestroyBody()
Fix a bug for positions of dropped items