Fix warning when compiling with -Wconversion
Fix warning when compiling with -Wconversion

This is not a real issue, but adding the cast to CGFloat silences this warning, which could appear for users of PureLayout if they have some uncommon build settings.

Fixes #73, fixes #49

yushuyi/PureLayout,sandyway/PureLayout,shnuzxw/PureLayout,neonaldo/PureLayout,adachic/PureLayout,imjerrybao/PureLayout,ernestopino/PureLayout,PureWiki/PureLayout,TongKuo/PureLayout,abhilashreddykallepu/PureLayout,jiamaozheng/PureLayout,yushuyi/PureLayout,liuyujing/PureLayout,yuanhoujun/PureLayout,foxswang/PureLayout,bruce-wmm/PureLayout,openaphid/PureLayout,kompozer/PureLayout,foxswang/PureLayout,Joneze/PureLayout,adachic/PureLayout,pivotal-oscar/PureLayout,lijie121210/PureLayout,Chunyulo/PureLayout,jiamaozheng/PureLayout,abhilashreddykallepu/PureLayout,FelixMLians/PureLayout,smileyborg/PureLayout,12207480/PureLayout,pivotal-oscar/PureLayout,lijie121210/PureLayout,bruce-wmm/PureLayout,neonaldo/PureLayout,fhchina/PureLayout,cnbin/PureLayout,cnbin/PureLayout,Chunyulo/PureLayout,FiftyThree/PureLayout,openaphid/PureLayout,k214k/PureLayout,eugenevinitsky/PureLayout,FiftyThree/PureLayout,ppamorim/PureLayout,pfleiner/PureLayout,tangwei6423471/PureLayout,sandyway/PureLayout,imjerrybao/PureLayout,12207480/PureLayout,ernestopino/PureLayout,TongKuo/PureLayout,PureWiki/PureLayout,fhchina/PureLayout,ppamorim/PureLayout,k214k/PureLayout,kompozer/PureLayout,tangwei6423471/PureLayout,liuyujing/PureLayout,Joneze/PureLayout,smileyborg/PureLayout