polish readme for yolov3

add GoogleNet support add quant low level api usage doc.

Elmo 1

* Add README.md

* Update url of images.

* Add images.

* Add usage.md

* Add demo.md

* Fix demo.md

* Add model_zoo.md

* Fix pretrain models download in model zoo.

* Refine documents.
1. Rename python to Python
2. Fix README.md: vgg->VGG, resnet->ResNet, mobilenet->MobileNet
3. Rename flops to FLOPS

* Move docs of PaddleSlim from models.fluid.PaddleSlim to models.PaddleSlim

* 1. Introduce model format in README.md
2. Merge results of combined strategies into one table.

* Refine README.md

* Refine README
1. Fix some terms in README.md
2. Fix description of model format in README.md
3. Add Channel-Wise into feature list
4. Rename 'paddle mobile' to 'Paddle Mobile'
5. Add comments for dynamic quantization and static quantization
6. Add entry url for Model Zoo in README.md

* Refine README.

fix yolov3 class_num.

fix - and _ mismatch, fix python3 bug in nonlocal codes

manually enable memory optimize for object_detection

* Fix name conficts in distillation.

* Download pretrain model in run_quant.sh

Paddle Video V2

Merge branch 'video_classification' of https://github.com/SunGaofeng/models into video_classification

fix weights/download.sh.

Remove core api in Transformer

* Add demo for combined strategies.

* Fix quan.

* Restore args in run.sh