Make Asciidoctor.js available to Bower

I add a bower.json file.
It's important to use [Semver](http://semver.org/) Git tags (cf [Bower documentation](https://github.com/bower/bower#registering-packages)).

Tag **v1.5.0.preview.1** does not conform, it should be **v1.5.0-preview.1** : 

```
anthonny@N550JV ~/99-Projects/test $ bower info https://github.com/anthonny/asciidoctor.js.git
bower cached        https://github.com/anthonny/asciidoctor.js.git#0.1.2
bower validate      0.1.2 against https://github.com/anthonny/asciidoctor.js.git#*
{
  name: 'asciidoctor.js',
  homepage: 'https://github.com/anthonny/asciidoctor.js',
  version: '0.1.2'
}
Available versions:
  - 1.5.0-preview.1
  - 0.1.2
You can request info for a specific version with 'bower info 
https://github.com/anthonny/asciidoctor.js.git#<version>'
```
Tag v1.5.0.preview.1 is not listed by Bower, it would be great to create a new tag **v1.5.0-preview.1**.

To register in public Bower repository :
```
bower register asciidoctor.js git://github.com/asciidoctor/asciidoctor.js.git
```