Add mutable streaming node example

This demo requires an input hooked up to the default recording device
and will stream that input to the default output device for 10 seconds,
alternating between a muted and unmuted state every second

examples/mutable_streaming_node.jl

@@ -0,0 +1,40 @@
+# This demos how real-time audio manipulation can be done using AudioNodes. To
+# run it, hook up some input audio to your default recording device and run the
+# script. The demo will run for 10 seconds alternating the node between a muted
+# and unmuted state
+using AudioIO
+
+type MutableNode <: AudioIO.AudioNode
+  active::Bool
+  deactivate_cond::Condition
+  muted::Bool
+
+  function MutableNode(muted::Bool)
+    new(false, Condition(), muted)
+  end
+end
+
+function MutableNode()
+  MutableNode(false)
+end
+
+import AudioIO.render
+function render(node::MutableNode, device_input::AudioIO.AudioBuf, info::AudioIO.DeviceInfo)
+  return device_input .* !node.muted, AudioIO.is_active(node)
+end
+
+function mute(node::MutableNode)
+  node.muted = true
+end
+
+function unmute(node::MutableNode)
+  node.muted = false
+end
+
+mutableNode = MutableNode()
+AudioIO.play(mutableNode)
+muteTransitions = { true => unmute, false => mute }
+for i in 1:10
+  sleep(1)
+  muteTransitions[mutableNode.muted](mutableNode)
+end