add think.prevent & think.isPrevent test case
add think.prevent & think.isPrevent test case

akira-cn/thinkjs,maxzhang/thinkjs,75team/thinkjs,snadn/thinkjs,maxzhang/thinkjs,akira-cn/thinkjs,75team/thinkjs,thinkjs/thinkjs,75team/thinkjs,Qihoo360/thinkjs,akira-cn/thinkjs,maxzhang/thinkjs