Kill the running process (group) on Get and Del.

Fixes #5.
Ditch the reflect stuff. It dirties the hell out of this code. The program should fail to compile if this feature isn't present.

Or if you don't want it to fail to compile for any OSes that Go currently supports, move all kill-related code to a separate file with `// +build !windows` and another file specific to windows, because that seems to be the only OS in src/syscall that has a different way of creating process groups. I can figure out the proper kill incantation for Windows later.