Merge pull request #169 from whuthm/master

fix bug :issue-39, when download a wallpaper, the wallpaper thumbnai applys the preview image.
Merge pull request #174 from whuthm/master

fix bug issue-41, when do not download online wallpaper ,the native wallpaper is null.
add logs for debug issue-8
Merge pull request #85 from heyongjian/master

huangming:merge the main branch to native branch.