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
fix incorrectly escaping ampersand

The bug is introduced in commit 175e04a862429903c17e5653e9fe1eda694354db, which quoted the URI. There is no need for escaping ampersand in quoted string, which only produces unwanted slash.
Merge pull request #40 from livibetter/fix-ampersand

fix incorrectly escaping ampersand
added space to internal comment
documented defaults for units and symbols
added sunrise and sunset data to the display
fetch_cmd variable already set in line-39.
standardize icon key style
Merge pull request #42 from royhodgman/doc

small documentation changes
Merge pull request #44 from thirota/fix-config-sample

standardize icon key style
Merge pull request #43 from thirota/onetime-fetch_cmd

fetch_cmd variable already set in line-39.
Adding Mist and Haze icons
Merge pull request #45 from thirota/mist

Adding Mist and Haze icons
uname is unused
Merge pull request #46 from thirota/unused-uname

uname is unused