This is a 1 command creation that allows you have a shadow player. You can switch between the shadow and yourself whenever you want, allowing you to play multiplayer maps by yourself. You use the signs to get the switching and shadow egg. Use the shadow egg to spawn the shadow initially, then use the switch egg to switch places with it whenever you want!
The download link is:
http://pastebin.com/TbxNxUs6

Installation Instructions:
Just give yourself a command block with /give @p command_block copy and paste the command into the command block. Then click on "Needs Redstone" to change it to "Always Active" and click "Done." That will install the command creation into your world