First pass at an event-based notification facilty

The matrix.eventable module/function returns a table with a pair of functions
which can be used to connect event handlers to events, and to generate them.
Basic usage is as follows:

  obj = eventable()
  obj.hook("event-name", print)  -- Handle the event with "print"
  obj.fire("event-name", "Hello, world")  -- Prints "Hello, world"

This initial implementation has the following limitations:

* Event handlers are always invoked in the same order in which they have been
  connected using the .hook() function.
* Individual event handlers cannot be disconnected. Only disconnecting all
  handlers for en event at once is possible via .hook("event", nil).

test/eventable.lua

@@ -0,0 +1,52 @@
+#! /usr/bin/env lua
+--
+-- eventable.lua
+-- Copyright (C) 2016 Adrian Perez <aperez@igalia.com>
+--
+-- Distributed under terms of the MIT license.
+--
+
+local eventable = require "matrix.eventable"
+
+do -- Simple event
+   local ev = assert(eventable())
+   local flag = false
+   ev.hook("foo", function () flag = true end)
+   ev.fire("foo")
+   assert(flag)
+end
+
+do -- Stop at first handler that returns some value
+   local ev = assert(eventable())
+   local flag1, flag2, flag3 = false, false, false
+   ev.hook("foo", function () flag1 = true end)
+   ev.hook("foo", function () flag2 = true ; return 42 end)
+   ev.hook("foo", function () flag3 = true ; return 0 end)
+   assert(ev.fire("foo") == 42)
+   assert(flag1 == true)
+   assert(flag2 == true)
+   assert(flag3 == false)
+end
+
+do -- Arguments to eventable() are passed to handlers
+   local obj = { answer = 42 }
+   local ev = assert(eventable(obj))
+   ev.hook("foo", function (o)
+      assert(o == obj)
+      assert(o.answer == 42)
+      o.answer = 0  -- Mutate
+   end)
+   ev.fire("foo")
+   assert(obj.answer == 0)
+end
+
+do -- Multiple arguments passed to eventable
+   local ev = assert(eventable(42, "bar", nil, { v=10 }))
+   ev.hook("foo", function (a, s, n, o)
+      assert(a == 42)
+      assert(s == "bar")
+      assert(n == nil)
+      assert(o.v == 10)
+   end)
+   ev.fire("foo")
+end