Add basework for opacity by category

A bug must first be fixed as the materials are not changed as they are
supposed to.
Merge branch 'transparency' into issue_19
Add first version of opacity per category working

This is working now on a per category basis, lots of things to fix:
    - Change category is not working
    - Strings are still some placeholders
    - Table borders
    - Change so that general sphere opacity affects the per category
      opacity.
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
Remove unused code from emperor.js
Merge pull request #110 from ElDeveloper/issue_19

Add opacity per category in visibility tab