XBlock SDK Workbench compatible mixin and scenarios

To run VideoXBlock workbench scenarios

Install XBlock SDK in parent directory:

git clone git@github.com:edx/xblock-sdk.git -b v0.2.0 ../xblock-sdk

Make sure VideoXBlock is installed into your environment:
```
make dev-install
```
Run Workbench:
```
../xblock-sdk/manage.py runserver
```
Go to http://localhost:8000/ in your browser.