Block Level Sharing in Galleries

![screen shot 2014-10-17 at 15 46 01](https://cloud.githubusercontent.com/assets/2236852/4680607/58efe2a0-560c-11e4-9c7e-233e8f4230e9.png)

1. Refactored block level sharing to abstract code for use on galleries and other content types in the future
2. Block level sharing is now available in galleries, including lightbox mode
:eyes: please use the global mediator for our clickstream, rather than binding to `document.body` directly.