Add OSX screenshot
In Linux, use zenity if Yad is not present
Mac OSX screen added - Merge branch 'feature/osx-screenshot'
Merge branch 'jaxbot-feature/osx-screenshot'
Informations about zenity and OSX screen added
Doc and informations updated:
- Correct command names, closes #10
- Add zenity part.
Symbolic update to v0.8:
- Add possibility to set hex colors in lowercase (g:vcoolor_lowercase =
  1), closes #11
Informations and doc file updated
Make hexCol in uppercase in Hex2Lit function