Change detection of BSD vs GNU date

Previous detection was checking for uname Linux/Debian,
but Debian can run in a GNU environment as well.

Ref: http://stackoverflow.com/questions/8747845/how-can-i-detect-bsd-vs-gnu-version-of-date-in-shell-script
Handle sunrise and sunset being unset

For "Walluf, Germany", the sunrise and sunset variables
were being left unset resulting in:
```
./ansiweather: line 211: [: 1385212168: unary operator expected
./ansiweather: line 211: [: 1385212168: unary operator expected
```
Merge pull request #37 from andreineculau/patch-2

Change detection of BSD vs GNU date
Merge pull request #38 from joshmoore/sunrise-fix

Handle sunrise and sunset being unset
Exiting with error when weather data cannot be fetch
Documentation update
Exiting with error when no weather data exist for a given location
Adding an .ansiweatherrc example file showing all available options
Merge remote-tracking branch 'upstream/master'