Make asciidoctor-fopub be a rubygem and add fopub wrapper script

With this pull request, you can install asciidoctor-fopub with the `gem` or `bundle`.

before merge (as of now)

```
gem 'asciidoctor-fopub', github: 'hnakamur/asciidoctor-fopub', branch: 'rubygems'
```

When you merge this pull request, it becomes simple as below.

```
gem 'asciidoctor-fopub', github: 'asciidoctor/asciidoctor-fopub'
```

Then, you can install asciidoctor-fopub with `bundle` command.

```
bundle install --path vendor/bundle --binstubs vendor/bin
```

Finally, you can run `fopub` with `./vendor/bin/fopub`.

Could you review this pull request?
Thanks,
Hiroaki