Added falling

[ifvictr]

This isn't the best way to add falling, but it's a start. This should be temporary.

[falkirks]

This isn't implemented in the correct way. If physics is implemented, it should be in SpecterPlayer so it appears as though the player is falling and not that a plugin is making the player fall.

[PEMapModder]

Moreover, it should be done with downwards acceleration instead of teleportation. Create another pull request with a better solution.

[ifvictr]

@falkirks @PEMapModder Okay, I'll think of another way.