Revert "Merge pull request #296 from biocore/revert-291-issue-271"

This reverts commit 4cccca84e3ac38aa391070fc39ddcba225090338, reversing
changes made to 3d275ffdb16565f5920d66754be110850242c921.
DOC: Add documentation about fixing #271
ENH: Fix problem with --colory_by

When nothing was passed in to --color_by, the default behaviour was to
keep all the columns in the mapping file. This was inconsistent with the
documented behavior, to remove unique columns, which has now been
corrected. See the comments in this patch.

Fixes #271
Merge branch 'master' of git://github.com/biocore/emperor into issue-271

Conflicts:
	ChangeLog.md
BUG: Fix with --add_unique_columns

If we requested the unique columns to be kept, then only the SampleID
would be retained, which was in turn useless.

This has been fixed.
BUG: Bad categories were added on plot creation

If you didn't pass anything with -b you would get all the categories,
whilst if you passed --add_unique_columns you would get only SampleID.
This has been fixed for good. :bug:
Merge pull request #306 from ElDeveloper/issue-271

BUG: Fix problem with --color_by
Merge branch 'master' of git://github.com/biocore/emperor into issue-299
ENH: Use different colormaps for coloring

Using chroma we can use colorbrewer colormaps for gradient coloring.
I've moved the choose-color function into the util.js module and more
importantly I have removed the dependency on THREE.Color class. We were
loosely using this class and converting back and forth everywhere, and
we didn't need to do this.