Clean setup file, create launch script

- Setup file: 'entry_point' is used in stead of non-existent
  'bin/hackertray' for scripts
- hackertray package init file
  - does not check if it is called as a script (which should not happen)
  - main() code is now replaced in HackerNewsApp.run()
  - main() instantiates the app and calls run() on it (previous roles
    grouped)
image: correct declaration + usage

- Declaration: setup.py declares the image a 'hackertray' data, gives
  its location and asks its inclusion
- appindicator looks for the picture in folders near itself, instead of
  /usr/bin (near the main executable) or the first entry of path.
Merge pull request #7 from brunal/statusicon

Closes #5
Closes #6
Thanks to @brunal