Modify homework models & fix some settings

environment.bat.example

environment.ps1.example

environment.sh.example

src/homework/migrations/0001_initial.py

src/homework/migrations/0002_auto_20180124_1453.py

src/homework/migrations/0003_auto_20180124_1818.py

src/homework/migrations/0004_auto_20180125_1817.py

src/homework/migrations/0004_auto_20180204_1209.py

src/homework/migrations/0005_auto_20180126_0135.py

src/homework/migrations/0005_auto_20180204_1525.py

src/homework/migrations/0006_auto_20180204_1601.py

src/homework/migrations/0007_merge_20180205_2021.py

src/homework/migrations/0007_merge_20180209_1734.py

src/homework/migrations/0008_auto_20180221_1135.py

src/homework/migrations/0009_merge_20180404_0825.py

src/homework/migrations/0010_auto_20180404_0828.py

src/homework/migrations/0011_auto_20180404_0853.py

src/homework/migrations/0012_auto_20180404_1253.py

src/homework/models.py

src/homework/myfields.py