Add bower.json

[creynders]

Allows for publishing on bower. If you accept the PR you need to do

# 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

[apragacz]

+1 This would allow to use this library easily in webpack. Currently it's difficult because the main file cannot be resolved via the main field in bower.json.