Slime grenades

When I first played scientist on vgstation, about a year ago, I was thrilled at all the possible use of slime extracts, and was very confused by the impossibility of placing slime cores in large chem grenades (which is apparently possible on some other servers according to some wiki I read ages ago I'm not even sure anymore).
Anyway, one year later here I am, coding that feature in the game myself. As my understanding of the game's code deepens, so does my ambition.

TL;DR: changelog
-you can now fit a slime extract in Large Chem Grenade casings
-you can now fit up to two slime extracts in Ex Grenade casings
-slime reactions that occur after a delay (gold/oil/orange/dark blue) will occur instantly if the reaction occured in a chem grenade (as soon as the grenade is activated)
-added a few show_message for when gold/silver/grey slimes extract reactions occur

As for how the grenades themselves work (I had to do quite a few changes to what happens when a grenade is primed), I found it would be clearer for most people to just make a big picture:

![slime grenade](https://cloud.githubusercontent.com/assets/7573912/4587892/057b0c58-5035-11e4-920b-b4be47aba8ad.png)