Add scaling by tab in the main GUI

This will contain one slider per each category in the mapping file.
Add updates to emperor.js and to the examples

This updated allows the scaling of the spheres on a per category basis,
a new callback has been added to respond to this behavior.

The callback from the slider takes a new value, it being the name of the
category.
Merge branch 'master' of git://github.com/qiime/emperor into issue_105
Merge pull request #111 from ElDeveloper/issue_105

Add a new tab for scaling per category