ENH: Move getDiscreteColor to the util.js module

The code has been removed from emperor.js and is now included inside
util.js plus tests have been added to check this function is working
correctly.
TST: Update ui.js tests

Apparently chrome is now adding a name space to all the DOM objects,
however this is not a practice being followed by Safari or Firefox,
shouldn't be something to worry about

If possible we should find a way to skip these tests in any case the
browser is not Chrome.