Unofficial Atom ebuild

- [x] provide an atom-shell **live** ebuild
- [x] provide an atom **live** ebuild 
- [x] apply MIT license

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Fixes to be done :

- [x] atom.sh script must be patch as atom-shell and atom are in separate directories
 
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Next steps: 

- QA:PreStrip : 
  - [ ] Get rid of libchromiumcontent.so binary from **dev-util/atom-shell**
  - [ ] Get rid of libffmpegsumo.so binary from **dev-util/atom-shell**

- QA:StackExec :
  - [ ] Get rid of node binary from from **app-editor/atom**