use new mechanism to name picture file

Because sometimes,urlopen timeout will happen,if program still use
downloadThread.preDownloadedCounts to name file,it will cause file
recover,for example:

now downloading : 2,3,4 pic preDownloadedCounts=4
then: 2's thread catch a URLError,timeout
this will cause file 2 be removed,and preDownloadCounts-1=3
if a new pic download thread start,it will use filename 4,recover
happened
Merge branch 'master' of https://github.com/linuxhenhao/SearchEnginePicDownloader