Adds the ability to inject chemicals into the atmospherics pipeline thro...

...ugh the following methods:

* Inject into canister and release
* Inject into canister and hook into a loop
* Inject into canister and transfer into an internals tank
* Inject into canister and share between canisters

Adds an industrial sized 'injector' to the atmospherics closets to facilitate dispersion of chemicals into the air-stream.

Plasma, Fuel and Thermite in the air will add to the flammability of the air, but not neccesarily ignite it.

99.5% of chemicals can be dispersed, and will react as intended, including viruses, acid, slime toxin, alcohol etc.
* A known "issue", but not exactly an issue, is that explosive reactions (EMP,Potassium etc) will be small.

Here are some previews:
![airmeat](https://cloud.githubusercontent.com/assets/3829126/5519094/54b1b34e-8997-11e4-9f19-cae3c5795f98.PNG)
![sparkles](https://cloud.githubusercontent.com/assets/3829126/5519095/6f1ba9b0-8997-11e4-854f-b9eb574670ef.PNG)
![slimeterrorism](https://cloud.githubusercontent.com/assets/3829126/5519184/2f4607b4-899b-11e4-9456-e8ebea468dfb.PNG)
Really not happy with this, all the other turf overlays (plmaster and slmaster) are created once (Here: https://github.com/tgstation/-tg-station/blob/c9c14feec6336e9e6f91dfd8326d68d526c08806/code/world.dm#L68)
and defined globally so that you don't have the same number of chem overlays as you do turfs.