optimize seen_materials checking performance by pre save it to an array
eager load materials with files
futher refine code
Merge pull request #252 from wangqiang1208/features/material-eager-loading

further optimisations for materials
re-run test cases when student submit again
Merge pull request #253 from wangqiang1208/bugfixing/auto-grading-not-refreshed-after-unsubmit

re-run test cases when student submit again
Merge branch 'development' of github.com:Coursemology/coursemology.org into Deploy