Modified model's relationship and added Category Model.
Update manager.py, import new model
Merge pull request #1 from shonenada/feature/model

Modified Models
Modified animate form and some style
Added nav action
Modified styles of index.
Modified layout.
Added course template.
renamed course_list.html
Refactor layout.
Added category.html
Update styles.
Added view file.
import course module in app.py
Refactor layout again
Modified struct of models.
Merge pull request #2 from shonenada/feature/model-fix

Modified struct of models.
Added subject html
add subject view.
Added clip page.