Merge pull request #108 from ElDeveloper/transparency

Fixes bug with transparency
Merge branch 'master' of git://github.com/qiime/emperor into issue_19
Add functionality for per-category opacity sliders

The visibility tab, now presents a slider for each category, each of
these categories controls the opacity for the spheres there defined.

The master opacity slider changes all these values at the same time.
Update emperor.js for all examples