add codes for course downloading in client,modify some method downloadfinishitem and downloadfinishdetail
merge from origin/release
add code for course downloading
Merge remote-tracking branch 'origin/release' into zhangjc
Merge remote-tracking branch 'origin/release' into zhangjc
add code to handle the situation of log directory does not exist
add codes for course download
Merge remote-tracking branch 'origin/release' into zhangjc
merge from origin/release