Added the portal shield.

Quick coding project based on sprites made by someone in the thread in response to a request for a PMMM (anime) reference item.
While inactive, just behaves the same as a regular item. Can be clicked to be made active, which will allow it to be used as a storage device and give the user the ability to draw from it.
Capable of storing and weapons and explosives when active. Acts as a pocket dimension, and so has no weight or size limits.
The random version spawns with some stuff already in there. You can drag it to remove an item from the list of stored things, or click to remove a random item with a priority of weapons over other things. Will deactivate itself after not being used for a little bit or being dropped.

![pocketshield](https://cloud.githubusercontent.com/assets/7339857/5273364/0a5c4368-7a7e-11e4-9fab-3553561bc047.png)

Has to be done, really.
Tried calling ...() but Byond can't handle the tree procs.
Copying code is also a no-no.