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
Would be nice to transition the opacity change on the buttons. Try something like: `    @include transition(opacity .25s linear); `