Added v() methods
Added v() methods

The v() methods were missing from the Tree interface, but in some cases they
can be quite useful.

donnfelker/timber,angcyo/timber,zhuxiaohao/timber,MaTriXy/timber,maxme/timber,vipulshah2010/timber,hejunbinlan/timber,leixinstar/timber,jsibbold/timber,hejunbinlan/timber,MichaelEvans/timber,ab271994/timber,robertschmid/timber,angcyo/timber,ab271994/timber,JakeWharton/timber,vipulshah2010/timber,msdgwzhy6/timber,zhuxiaohao/timber,MaTriXy/timber,MaTriXy/timber,JakeWharton/timber,lockerfish/timber,donnfelker/timber,lockerfish/timber,leixinstar/timber,maxme/timber,msdgwzhy6/timber,jsibbold/timber,MichaelEvans/timber,jianlei/timber,leixinstar/timber,jianlei/timber,robertschmid/timber