Silicon Medical and Security HUDs!

- The AI can now access both Security and Medical HUDs via a new button
on its UI. The Medical HUD is exactly equal to all others. The AI's
Security HUD cannot detect implants.

- Cyborg HUD modules have been removed in favor of an inbuilt command,
to make it less of a hassle to access them.

- HUD code has been given its own file such that it can be used by any
mob. In addition, HUD users are placed into a list instead of searching
for only humans and checking them for a HUD item. This is to make it
easier to expand.

- Security HUD messages can now be received by any mob using a SecHUD. The icon is now a set of handcuffs.

New AI button and Sec HUD message: 
![screenshot 2014-08-21 23 03 01](https://cloud.githubusercontent.com/assets/4955510/4006651/7b73f9f6-29b1-11e4-9069-439b853a0913.png)

*I plan to expand the HUD broadcast feature to Medibots as part of another PR, as bot code is outside the scope of this one.*
Is sensor_mode() not a Silicon proc? because if it is, there's no need to individually typecast for AI and Borgs.