* add class aware sampling strategy
* remove redundancy code

# Add VGG-SSD on VOC and COCO dataset.
# Add config and model zoo. 
# Refine bbox2out and draw_bbox:
    - Add bbox de-normalize in bbox2out function.
    - Remove bbox de-normalize in draw_bbox.

* add save best model & fix dcn configs

* fix easy problem

* add rpn recall function and class-aware rpn

* optimize error message when result is none

* Rename object_detection to PaddleDetection
* Small fix for doc