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

[StevenLooman]

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.

[perplexes]

This is exactly what I was looking for -- thank you @StevenLooman !!

Note to maintainers: You've got some outstanding pull requests. I recommend pulling this one in, at least.

[Ariel-Rodriguez]

@perplexes @StevenLooman it fails

FAIL

./semver.sh 1.0.0-rc.1 gt 1.0.0-rc.0
./semver.sh: line 38: [: 1.1: integer expression expected
./semver.sh: line 42: [: 1.1: integer expression expected
./semver.sh: line 47: [: 0.1: integer expression expected
./semver.sh: line 51: [: 0.1: integer expression expected
./semver.sh: line 56: [: 0.1: integer expression expected
./semver.sh: line 60: [: 0.1: integer expression expected
0

tested under OSX

I created a similar tool simplified way that actually works as intended https://gist.github.com/Ariel-Rodriguez/9e3c2163f4644d7a389759b224bfe7f3