VIS: added ability to plot DataFrames and Series with errorbars
VIS: added broadcasting of error series across several data series, also added ability to pass a dictionary of errors with key as data series label and value as error values
VIS: added xerror to bar plots, added tests for broadcasting errors
VIS: added support for asymmetrical error bars
VIS: when kind='bar', default error bar color is set as the current mpl.rcParams['xtick.color'] instead of black for a bit more aesthetic consistency
changes to allow specifying errors for only some of the columns in a dataframe