Add option to change the coloring of elements

getColorList() has been changed to work using discrete or continuous
coloring. In order to support discrete coloring, a new function was
added (getDiscreteColor), this function will take a color from the
available list of QIIME Colors.

A callback has been added for a controller that will switch between
continuous or discrete coloring.
Add GUI element to switch coloring schemes

This new element allows the user to switch between continuous and
discrete coloring.

This fixes #30,
Merge pull request #65 from ElDeveloper/issue_30

Add option to change the coloring of elements -- fixes 30