Add a parameter to cLevel_Player::DownGrade_Player() for ignoring invincibility.

Previously, calling contexts had to adjust m_invincible manually
to 0 if they wanted to ignore invincibility and have Maryo die
regardless.
Make Maryo always die on lava, even if invincible. Fixes #51.

Now Maryo even dies when he has star effect or is shorttime-invincible
after being hurt.

Also, when hitting lava in God mode, you now get thrown up just
the same as when hitting the level bottom edge.
Add Player#kill to mruby scripting interface, make Player#kill! the more enforcing variant.