Add Bower Support

This adds bower support for Open Seadragon. To finalize this, two steps are required to be executed by the repo owner:

* Unregister the old package using:

  ```curl -X DELETE "https://bower.herokuapp.com/packages/OpenSeadragon?auth_token=TOKEN"```
  
  Detailed instructions: https://github.com/bower/bower/issues/120
* Register this new package using:

  ```bower register OpenSeadragon git://github.com/openseadragon/site-build```

Discussion is here:
https://github.com/openseadragon/openseadragon/pull/340