Make the ship invulnerable by passing 1
import("Game").player:SetInvulnerable(0)
import("Game").player:SetInvulnerable(1)
make Ship:SetInvulnerable take a boolean, not an integer
expand documentation for the Ship:Get/SetInvulnerable methods
move Ship:Get/SetInvulnerable methods out of 'AI Methods' doc section

Move Lua Ship Get/SetInvulnerable method implementations up the file,
so they aren't grouped with AI methods in the documentation.
document the fact that ship invulnerability is not saved
Merge remote-tracking branch 'fluffy/IAmInvincible'
fix inverted early-out test in Ship::AIGetLeadDir

Thanks to @walterar for noticing this bug.