Tweak date/credits

LICENSE

@@ -1,6 +1,6 @@
 The MIT License (MIT)
 
-Copyright (c) 2014 Omar Cornut
+Copyright (c) 2014-2015 Omar Cornut and ImGui contributors
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal

imgui.cpp

@@ -2,7 +2,7 @@
 // See ImGui::ShowTestWindow() for sample code.
 // Read 'Programmer guide' below for notes on how to setup ImGui in your codebase.
 // Get latest version at https://github.com/ocornut/imgui
-// Developed by Omar Cornut and contributors.
+// Developed by Omar Cornut and ImGui contributors.
 
 /*