BUG: Fix color selection in andrews curve"

fixed bug introduced in b5265a5b84ae1aeaf5fff88f1f9e5872a9b404e6
which change from picking a single random color for the class to
picking a random color for each line.
Merge pull request #5378 from tacaswell/fix_andrews_colors

BUG: Fixes color selection in andrews_curve
ENH: Add mode method to Series and DataFrame

Abstract building up the tables with counts so value_count and mode can
share. Aww!
DOC: Add documentation about mode()
better doc