Backpack watertank with nozzle [MAP]

Adds a backpack watertank with a detachable nozzle. One is available in the hydroponics store room.

* Contains 500 units
* Pours in 25, 50 or 100 units
* Has a detachable nozzle to pour/refill with
* Can only be poured/refilled when on a player's back
* Slows the player down (As much as an engineering hardsuit) - This PR includes slowdown addition from back items
* Nozzle acts as a proxy reagent holder to the watertank's reagents, so a permanent state of it is kept even though the nozzle disappears from the player's hands

Thankyou to validsalid for the wonderful sprites.

![](http://i.imgur.com/k7vIp1b.png)