Going down the Ferocity route is very common for Adc's as it increases attack damage and vamp, along with armour.
We go up the Cunning tree a bit to pick up TotalBiscuits and Dangerous Game to restore some health after kills/assists.
Back to Ferocity we get feast to keep our health up while farming, ensuring that we don't leave lane until we have enough to get a decent amount of items. We get Bounty hunter so we can get some extra damage as the game progresses, but remember you have to get the KILLS, assists DO NOT count.
Finally we get Warlord's bloodlust because of our high crit chance into the late game making us gain usually about 100 hp per crit once we're nearly done with our build.