Enable multiple users

Currently, it is not possible to use atom for multiple users on the same linux machine. The socket file '/tmp/atom.sock' belongs to the first user that starts atom.
With this small change, a socket file will be created for each user.

src/browser/atom-application.coffee

@@ -18,7 +18,7 @@ DefaultSocketPath =
   if process.platform is 'win32'
     '\\\\.\\pipe\\atom-sock'
   else
-    path.join(os.tmpdir(), 'atom.sock')
+    path.join(os.tmpdir(), "atom_#{process.env['USER']}.sock")
 
 # The application's singleton class.
 #