Add bower.json

Allows for publishing on bower. If you accept the PR you need to do 

```shell
# tag the release
git tag -a v0.9.1 -m 'Release v0.9.1` && git push origin v0.9.1
# install bower
npm install -g bower
# register with bower
bower register backbone.bootstrap-modal git://github.com/powmedia/backbone.bootstrap-modal
```

Bower uses the tags on GH, so you need to tag it first.

Fix #60