Implement the ipc 'binding' event

The binding event will be triggered when a binding is run as a result of
some a user action. The binding event has the following properties:

change: (str) Currently this will only be "run" but may be expanded in
the future. Included for consistency with other events.

binding: (map) the serialized binding

The "binding" member will have these properties:

input_type: (str) either "keyboard" or "mouse"

input_code: (int) the xcb keycode of the keyboard binding if it was
provided or the mouse button if it is a mouse binding.

symbol: (str) the string representation of the input code

command: (str) the bound command

mods: (list of str) a list of the modifiers that were pressed as string
symbols

fixes #1210

docs/ipc

@@ -1,7 +1,7 @@
 IPC interface (interprocess communication)
 ==========================================
 Michael Stapelberg <michael@i3wm.org>
-February 2014
+October 2014
 
 This document describes how to interface with i3 from a separate process. This
 is useful for example to remote-control i3 (to write test cases for example) or
@@ -638,6 +638,9 @@ window (3)::
 barconfig_update (4)::
     Sent when the hidden_state or mode field in the barconfig of any bar
     instance was updated and when the config is reloaded.
+binding (5)::
+	Sent when a configured command binding is triggered with the keyboard or
+	mouse
 
 *Example:*
 --------------------------------------------------------------------
@@ -749,6 +752,47 @@ This event consists of a single serialized map reporting on options from the
 barconfig of the specified bar_id that were updated in i3. This event is the
 same as a +GET_BAR_CONFIG+ reply for the bar with the given id.
 
+=== binding event
+
+This event consists of a single serialized map reporting on the details of a
+binding that ran a command because of user input. The +change (sring)+ field
+indicates what sort of binding event was triggered (right now it will always be
++"run"+ but may be expanded in the future).
+
+The +binding (object)+ field contains details about the binding that was run:
+
+command (string)::
+	The i3 command that is configured to run for this binding.
+mods (array of strings)::
+	The modifier keys that were configured with this binding.
+input_code (integer)::
+	If the binding was configured with +bindcode+, this will be the key code
+	that was given for the binding. If the binding is a mouse binding, it will be
+	the number of the mouse button that was pressed. Otherwise it will be 0.
+symbol (string)::
+	If this is a keyboard binding that was configured with +bindsym+, this
+	field will contain the given symbol.
+input_type (string)::
+	This will be +"keyboard"+ or +"mouse"+ depending on whether or not this was
+	a keyboard or a mouse binding.
+
+*Example:*
+---------------------------
+{
+ "change": "run",
+ "binding": {
+  "command": "nop",
+  "mods": [
+    "shift",
+    "ctrl"
+  ],
+  "input_code": 0,
+  "symbol": "t",
+  "input_type": "keyboard"
+ }
+}
+---------------------------
+
 == See also (existing libraries)
 
 [[libraries]]