ENH: Add styling for drop-down menus in tabs

This styling will fix the problems that would occurr when category names
were too long for the tab to be displayed, resulting in a mal-formed
layout.

Fixes #246
ENH: Use emperor-tab-drop-down class
DOC: Add note to ChangeLog.md

About resolving the problem with long category names.
TST: Change mapping file to have long names
Merge pull request #259 from ElDeveloper/issue-246

Fix problem with long metadata category names