Code Refactor, add webcam demo...lots of stuff

[clement10601]

first of all, plz ignore my bad commit and coding style...m(_ _)m

1. yolo3_one_file_to_detect_them_all.py modifed and add a new script yolo3_cam.py which can use webcam as source and send every frame to yolo3 prediction (pretty slow on mac book pro, less than 1 fps)

2. add model module
   from model.yolo3 import make_yolov3_model
   I am working on something like:
   class yolo3(nn.Modules)
   and make importable by folling command
   from model import yolo3

3. change demo script import functions

from utils.bbox import BoundBox
from utils.tools import preprocess_input, decode_netout
from utils.tools import correct_yolo_boxes, do_nms, draw_boxes