Extended semver.sh to also work as a stand alone command + rewritten tests

Extended semver.sh to also work as a stand alone command. Now one can do:
```
  $ semver.sh 1.0.0 gt 1.0.1
  1
  $ semver.sh 1.0.0 lt 1.0.1
  0
```

Also, reimplemented the tests to make it easier to spot errors.