Support for custom css class for backdrops.

I had a use-case in my project where I needed the backdrops to have different types of visuals, and I thought I would contribute this change to this project.

To use this feature, simple add data-backdrop-class="" to your modal element and it will append the class you specified on to the created backdrop.

I would like to say that this effort is fantastic, for our project it's solved a few things and saving lots of time. Great initiative and keep up the good work.

I've taken the liberty to rename components.json to bower.json and updated the repo's version. Bower was complaining about the version string "2.1", changing it to "2.1.0" solved it for me.