[MRG] Fix classification_report_imbalanced output_dict keys to target_names

[AYY7]

Reference Issue

Fixes #863

What does this implement/fix? Explain your changes.

When output_dict=True and target_names is given, set the keys of the output_dict to the corresponding target_names

{'label 1': {'pre':0.5,
             'rec':1.0,
             ...
            },
 'label 2': { ... },
  ...
}

When output_dict=True and target_names is not given, set the keys of the output_dict to the corresponding labels in str format

{'0': {'pre':0.5,
             'rec':1.0,
             ...
            },
 '1': { ... },
  ...
}

Any other comments?