start develop portable version atom

If an .atom folder is present within the installation directory it will be used.
#2939,  #3320 
next steps:
- add --portable flag to build script
- update https://github.com/atom/atom/blob/master/src/config.coffee#L700, copy from dot-atom to portable folder, when folder missing. Mayby add to core settings portable flag when atom build? 

how you look at it?