Add background color in the GUI

A color palette is now available at the bottom of the 'Options' tab;
default color is now black. The palette has offers black and white as
options but about every color with the manual color picker.

This fiexes #9
Merge branch 'master' of git://github.com/qiime/emperor into issue_9
Change color formatting to always be HEX6

Not having this formatting in place was causing that some colors i. e.
FFFFFF would be converted into FFF which understood as an HEX6 is a blue
tone not white.
Fixes tests in test_format.py

This pull request was not accounting for the formatting in the
HTML_FOOTER constant.

Fixes #9
Merge pull request #64 from ElDeveloper/issue_9

Add background color in the GUI