Use domain sockets to connect to R processes

This provides better performance and, more importantly, better security
versus the previous TCP socket implementation.
Merge branch 'master' into domain-sockets

Conflicts:
	lib/proxy/http.js
	lib/worker/app-worker.js
	lib/worker/worker-registry.js
Move responsibility of handling worker acquisition to a "Scheduler."

Initially just implementing a simple scheduler that replicates the previous behaviour.
Use pkill and SIGABRT in a Mac environment so R is successfully killed.
Merged changes from osx-kill
Create placeholder for test.
Added agpl headers to the tests.
Added ability to direct traffic to a particular worker.
Merged poly-scheduler with master
Merged domain-sockets branch in.