Run all task spawning through util, to allow for easy hooking.

During debugging, I found it useful to hook all task creation in a
central location, and util::task was the perfect place for it.

r? @pcwalton (or maybe someone else, I'm kinda sending you a bunch of
reviews today because I don't know who better to give them to)
make naming more consistent
auto merge of #3827 : cgaebel/servo/use-custom-spawn, r=pcwalton

During debugging, I found it useful to hook all task creation in a
central location, and util::task was the perfect place for it.

r? @pcwalton (or maybe someone else, I'm kinda sending you a bunch of
reviews today because I don't know who better to give them to)