Add Data Labels to GroupedBar Chart

This adds data labels to the GroupedBar chart. Added additional parameters data_labels (boolean to turn on/off), color, fontsize, and baseline (vertical alignment). 

Examples:
group = vincent.GroupedBar(df_2, data_labels= True, label_color='#000000', fontsize= 11)
group2 = vincent.GroupedBar(df_2[:3], data_labels= True, label_color='#000099', fontsize= 18, baseline='bottom')