Calculate load time in index.js

Do it previously in window-bootstrap caused several things to not be
included such as requiring coffee script and atom shell modules.

Now it is a much more accurate representation of on load time.

src/window-bootstrap.coffee

@@ -1,14 +1,10 @@
 # Like sands through the hourglass, so are the days of our lives.
-startTime = Date.now()
-
 require './window'
 
 Atom = require './atom'
 window.atom = Atom.loadOrCreate('editor')
 atom.initialize()
 atom.startEditorWindow()
-window.atom.loadTime = Date.now() - startTime
-console.log "Window load time: #{atom.getWindowLoadTime()}ms"
 
 # Workaround for focus getting cleared upon window creation
 windowFocused = ->