Merge remote-tracking branch 'origin/release' into zhangjc
merge from origin/release
add forms of feedback,version check,topmost in setting, and http request interactive with server
Thank God for successfully recovery previous work, which is fixing bugs in login, adding local cache for username and password
add recent learing and recent downloading in setting
merge from origin/release
add a function for getting download file length
Merge remote-tracking branch 'origin/zhangjc' into release
doc 80% done