Added bower.json package manager configuration file

Hi there,

First of all, thanks for your great work on po2json!

I created [gettext.js](https://github.com/guillaumepotier/gettext.js) that rely on your lib to dump its .json translated files (a little bit modified though, to have the smallest json file possible, your formats raw and Jed had IMO unnecessary characters) and it works fine with npm (I declared your project as dependency) but not currently with bower. Since your project does not have a bower configuration, in mine, I cannot declare yours as dependance.

Could be great if you could merge this PR with this small config file and register the plugin doing: 

```shell
bower register <my-package-name> <git-endpoint>
# for example
$ bower register example git://github.com/user/example.git
```

Nothing to do then, Bower auto-updates when you create git tags.

Thanks a lot!

Best