Added support for reading the LANG environment variable to determine the language

getLanguageFromEnvironment in Aura/Languages.hs defines currently supported
languages and fall backs to English if neither the command line argument is supplied
nor there is a $LANG environment variable. It uses pattern matching (serbian is
matched with ('s':'r':xs) for example) which might be dificult for non-haskellers, but
that was the most straight forward solution to xx_YY@zzz.www style language codes
with most parts being optional.
`NotSpecified` value in `Language` removed

- It was a runtime error waiting to happen. I've altered it so that
  `Maybe Language` values are used instead.
- This fixes potential runtime errors gets rid of the wall of compile-time
  warnings, but introduces a small bug: `aura -h` will always be displayed
  in English if no language flag is specifically given.
Merge branch 'brcha-master'