Gemspec for gitflowversion

As discussed with @andreasohlund I provide in this pull request a gemspec which allows to push gitflowversion to rubygems. As soon as the pull is accepted I will push it to the gallery. Please review the gemspec carefully whether the provided information is correct.

I will then add someone of you as the coowner of the package. Building the package is straight forward

go to the directory

```
gem build gitflowversion.gemspec
gem push gitflowversion-VERSION.gem
```