update achievement gain date to submission date(not grading date)
refine code
change activerecord code to sql statement
refine code
Merge remote-tracking branch 'upstream/development' into bugfixing/achievement
Using new obtained_at field
set obtained_at after create
run migration
add obtained_at in sql statement
add migrate file
add rake file to set obtain_at date
changed call back to before_create
set obtained at to submit date
remove debugging code '
using class function to assign date
Merge pull request #204 from wangqiang1208/bugfixing/achievement

update achievement gain date to submission date(not grading date)
fixing question nil issue in code_of_last_attempt
Merge pull request #209 from wangqiang1208/features/auto-save-training

fixing question nil issue in code_of_last_attempt
change grading.save to grading.update_column
the save function is in the save callback previously.
add comments of the usage of exp_crate_or_changed in update_exp_transaction