[MAP] Added a new gravity generator which will produce gravity for the station.

Sprites by Ausop, which he posted in his sprite thread.

![](http://i.imgur.com/aUN82Y9.png)

**Gravity Generator**
* The gravity generator is very sturdy, as it is almost indestructible, but explosions can break it and it can be repaired.
* You cannot turn the machine on or off immediately, as it has to first charge or discharge, which will take a minute.
* While charge or discharging it will give off a lot of radiation, requiring protective clothing. It will also make sounds and change the green energy overlay in the middle to indicate this to players.
* When a gravity generator has been fully charged or discharged, if it has managed to change the z level's gravity it will then shake the screens of everyone in the level and play a noise.
* Added the gravity generator room to the west of Engineering. It is very secure, with a foyer and requiring CE or Captain access. There are instructions on the table for repairing and enabling/disabling the machine.

**Misc**
* Changed C4 to stop del()'ing and instead will only use ex_act().
* Added an interact_offline variable for machines that will allow you to interact with them even if they are depowered. The gravity generator only uses this at the moment.
* Removed the stun that people get when gravity changes.
* Shuttles, the DJ station and the mining asteroid has gravity still. The derelict doesn't.
* Someone wearing magboots in zero-g can walk on floor tiles normally.