* [Fix] Fix example
* rename Net to Model
* support training without gpu in example
* modify according to comment
* replace data path with relative path
* Assign different priority to default hooks, and add custom hook register in base runner.
* Add custom hook register in example train file
* Add unittest of custom hook
* Code format
* add yapf and isort to travis
* minor formatting
* remove blank lines
* skip unit tests for progressbar when python2
* update travis to ubuntu 16.04
* use a newer version ffmpeg
* add -y to add-apt-repository