Allow wrapping of all registered callbacks with global wrapper.

[creationix]

This depends on #127.

Once global error-handling is implemented, we will add global event wrapping. Every place where C accepts a lua function to be used as a callback later on, it will be optionally replaced with a new function (usually a wrapped version). This is useful for event tracing, long stack traces and other diagnostics related to event chains and trees.

The global callback wrapper/mutator will be set the same way as the global error handler in #127, but as a third arg to uv.run(mode, onError, onRegister)

The onRegister(type, scope, fn) function will return an optional function to replace the passed in fn. For example, a simple logger that logs all timer setups and timeout will look like:

uv.run(nil, nil, function (type, scope, fn)
  -- Ignore non-timer events
  if type ~= "timer" then return end
  print("setup", scope)
  return function (...)
    print("emit", scope)
    return fn(...)
  end
end)

local timer = uv.new_timer()
timer:start(100, 100, function ()
  print("tick")
end)

This would print setup uv_timer_t: 0x41bc5c60 before timer:start() returned and then later on every interval tick would print emit uv_timer_t: 0x41bc5c60 before running the callback that prints tick.